diff --git a/src/Microsoft.AspNetCore.Mvc.Razor/TagHelpers/UrlResolutionTagHelper.cs b/src/Microsoft.AspNetCore.Mvc.Razor/TagHelpers/UrlResolutionTagHelper.cs
index e9999ece69..ec5cf66f90 100644
--- a/src/Microsoft.AspNetCore.Mvc.Razor/TagHelpers/UrlResolutionTagHelper.cs
+++ b/src/Microsoft.AspNetCore.Mvc.Razor/TagHelpers/UrlResolutionTagHelper.cs
@@ -157,51 +157,52 @@ namespace Microsoft.AspNetCore.Mvc.Razor.TagHelpers
                 throw new ArgumentNullException(nameof(output));
             }
 
-            IReadOnlyList<TagHelperAttribute> attributes;
-            if (output.Attributes.TryGetAttributes(attributeName, out attributes))
+            for (var i = 0; i < output.Attributes.Count; i++)
             {
-                for (var i = 0; i < attributes.Count; i++)
+                var attribute = output.Attributes[i];
+                if (!string.Equals(attribute.Name, attributeName, StringComparison.OrdinalIgnoreCase))
                 {
-                    var attribute = attributes[i];
-                    var stringValue = attribute.Value as string;
-                    if (stringValue != null)
+                    continue;
+                }
+
+                var stringValue = attribute.Value as string;
+                if (stringValue != null)
+                {
+                    string resolvedUrl;
+                    if (TryResolveUrl(stringValue, resolvedUrl: out resolvedUrl))
                     {
-                        string resolvedUrl;
+                        output.Attributes[i] = new TagHelperAttribute(attribute.Name, resolvedUrl);
+                    }
+                }
+                else
+                {
+                    var htmlContent = attribute.Value as IHtmlContent;
+                    if (htmlContent != null)
+                    {
+                        var htmlString = htmlContent as HtmlEncodedString;
+                        if (htmlString != null)
+                        {
+                            // No need for a StringWriter in this case.
+                            stringValue = htmlString.ToString();
+                        }
+                        else
+                        {
+                            using (var writer = new StringWriter())
+                            {
+                                htmlContent.WriteTo(writer, HtmlEncoder);
+                                stringValue = writer.ToString();
+                            }
+                        }
+
+                        IHtmlContent resolvedUrl;
                         if (TryResolveUrl(stringValue, resolvedUrl: out resolvedUrl))
                         {
-                            attribute.Value = resolvedUrl;
+                            output.Attributes[i] = new TagHelperAttribute(attribute.Name, resolvedUrl);
                         }
-                    }
-                    else
-                    {
-                        var htmlContent = attribute.Value as IHtmlContent;
-                        if (htmlContent != null)
+                        else if (htmlString == null)
                         {
-                            var htmlString = htmlContent as HtmlEncodedString;
-                            if (htmlString != null)
-                            {
-                                // No need for a StringWriter in this case.
-                                stringValue = htmlString.ToString();
-                            }
-                            else
-                            {
-                                using (var writer = new StringWriter())
-                                {
-                                    htmlContent.WriteTo(writer, HtmlEncoder);
-                                    stringValue = writer.ToString();
-                                }
-                            }
-
-                            IHtmlContent resolvedUrl;
-                            if (TryResolveUrl(stringValue, resolvedUrl: out resolvedUrl))
-                            {
-                                attribute.Value = resolvedUrl;
-                            }
-                            else if (htmlString == null)
-                            {
-                                // Not a ~/ URL. Just avoid re-encoding the attribute value later.
-                                attribute.Value = new HtmlString(stringValue);
-                            }
+                            // Not a ~/ URL. Just avoid re-encoding the attribute value later.
+                            output.Attributes[i] = new TagHelperAttribute(attribute.Name, new HtmlString(stringValue));
                         }
                     }
                 }
diff --git a/src/Microsoft.AspNetCore.Mvc.TagHelpers/ImageTagHelper.cs b/src/Microsoft.AspNetCore.Mvc.TagHelpers/ImageTagHelper.cs
index eafde361b9..498551204d 100644
--- a/src/Microsoft.AspNetCore.Mvc.TagHelpers/ImageTagHelper.cs
+++ b/src/Microsoft.AspNetCore.Mvc.TagHelpers/ImageTagHelper.cs
@@ -106,7 +106,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 // not function properly.
                 Src = output.Attributes[SrcAttributeName].Value as string;
 
-                output.Attributes[SrcAttributeName] = _fileVersionProvider.AddFileVersionToPath(Src);
+                output.Attributes.SetAttribute(SrcAttributeName, _fileVersionProvider.AddFileVersionToPath(Src));
             }
         }
 
diff --git a/src/Microsoft.AspNetCore.Mvc.TagHelpers/InputTagHelper.cs b/src/Microsoft.AspNetCore.Mvc.TagHelpers/InputTagHelper.cs
index e20e8557ed..f347977d78 100644
--- a/src/Microsoft.AspNetCore.Mvc.TagHelpers/InputTagHelper.cs
+++ b/src/Microsoft.AspNetCore.Mvc.TagHelpers/InputTagHelper.cs
@@ -183,7 +183,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             // inputType may be more specific than default the generator chooses below.
             if (!output.Attributes.ContainsName("type"))
             {
-                output.Attributes["type"] = inputType;
+                output.Attributes.SetAttribute("type", inputType);
             }
 
             TagBuilder tagBuilder;
diff --git a/src/Microsoft.AspNetCore.Mvc.TagHelpers/Internal/AttributeMatcher.cs b/src/Microsoft.AspNetCore.Mvc.TagHelpers/Internal/AttributeMatcher.cs
index cfe3496c3c..5caa08dc5b 100644
--- a/src/Microsoft.AspNetCore.Mvc.TagHelpers/Internal/AttributeMatcher.cs
+++ b/src/Microsoft.AspNetCore.Mvc.TagHelpers/Internal/AttributeMatcher.cs
@@ -73,7 +73,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers.Internal
             // Perf: Avoid allocating enumerator
             for (var i = 0; i < requiredAttributes.Length; i++)
             {
-                IReadOnlyTagHelperAttribute attribute;
+                TagHelperAttribute attribute;
                 if (!context.AllAttributes.TryGetAttribute(requiredAttributes[i], out attribute))
                 {
                     // Missing attribute.
diff --git a/src/Microsoft.AspNetCore.Mvc.TagHelpers/LinkTagHelper.cs b/src/Microsoft.AspNetCore.Mvc.TagHelpers/LinkTagHelper.cs
index 202d7c3571..c733fe11f8 100644
--- a/src/Microsoft.AspNetCore.Mvc.TagHelpers/LinkTagHelper.cs
+++ b/src/Microsoft.AspNetCore.Mvc.TagHelpers/LinkTagHelper.cs
@@ -254,7 +254,10 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
 
                 if (Href != null)
                 {
-                    output.Attributes[HrefAttributeName].Value = _fileVersionProvider.AddFileVersionToPath(Href);
+                    var index = output.Attributes.IndexOfName(HrefAttributeName);
+                    output.Attributes[index] = new TagHelperAttribute(
+                        HrefAttributeName,
+                        _fileVersionProvider.AddFileVersionToPath(Href));
                 }
             }
 
@@ -303,7 +306,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     continue;
                 }
 
-                attributes[HrefAttributeName] = url;
+                attributes.SetAttribute(HrefAttributeName, url);
                 BuildLinkTag(attributes, builder);
             }
         }
diff --git a/src/Microsoft.AspNetCore.Mvc.TagHelpers/ScriptTagHelper.cs b/src/Microsoft.AspNetCore.Mvc.TagHelpers/ScriptTagHelper.cs
index 7a2d564e71..2935adb100 100644
--- a/src/Microsoft.AspNetCore.Mvc.TagHelpers/ScriptTagHelper.cs
+++ b/src/Microsoft.AspNetCore.Mvc.TagHelpers/ScriptTagHelper.cs
@@ -217,7 +217,10 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
 
                 if (Src != null)
                 {
-                    output.Attributes[SrcAttributeName].Value = _fileVersionProvider.AddFileVersionToPath(Src);
+                    var index = output.Attributes.IndexOfName(SrcAttributeName);
+                    output.Attributes[index] = new TagHelperAttribute(
+                        SrcAttributeName,
+                        _fileVersionProvider.AddFileVersionToPath(Src));
                 }
             }
 
@@ -267,7 +270,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     continue;
                 }
 
-                attributes[SrcAttributeName] = url;
+                attributes.SetAttribute(SrcAttributeName, url);
                 BuildScriptTag(attributes, builder);
             }
         }
diff --git a/src/Microsoft.AspNetCore.Mvc.TagHelpers/TagHelperOutputExtensions.cs b/src/Microsoft.AspNetCore.Mvc.TagHelpers/TagHelperOutputExtensions.cs
index c938ba3778..dcd917675b 100644
--- a/src/Microsoft.AspNetCore.Mvc.TagHelpers/TagHelperOutputExtensions.cs
+++ b/src/Microsoft.AspNetCore.Mvc.TagHelpers/TagHelperOutputExtensions.cs
@@ -116,7 +116,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
 
                     if (tagHelperOutput.Attributes.TryGetAttribute("class", out classAttribute))
                     {
-                        tagHelperOutput.Attributes["class"] = classAttribute.Value + " " + attribute.Value;
+                        tagHelperOutput.Attributes.SetAttribute("class", classAttribute.Value + " " + attribute.Value);
                     }
                     else
                     {
@@ -158,12 +158,10 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             TagHelperContext context)
         {
             var existingAttribute = context.AllAttributes[allAttributeIndex];
-            var copiedAttribute = new TagHelperAttribute
-            {
-                Name = existingAttribute.Name,
-                Value = existingAttribute.Value,
-                Minimized = existingAttribute.Minimized
-            };
+            var copiedAttribute = new TagHelperAttribute(
+                existingAttribute.Name,
+                existingAttribute.Value,
+                existingAttribute.Minimized);
 
             // Move backwards through context.AllAttributes from the provided index until we find a familiar attribute
             // in tagHelperOutput where we can insert the copied value after the familiar one.
diff --git a/test/Microsoft.AspNetCore.Mvc.Razor.Test/RazorPageTest.cs b/test/Microsoft.AspNetCore.Mvc.Razor.Test/RazorPageTest.cs
index b9646a956e..0ebf873016 100644
--- a/test/Microsoft.AspNetCore.Mvc.Razor.Test/RazorPageTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.Razor.Test/RazorPageTest.cs
@@ -978,7 +978,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             page.EndAddHtmlAttributeValues(executionContext);
 
             // Assert
-            var htmlAttribute = Assert.Single(executionContext.HTMLAttributes);
+            var htmlAttribute = Assert.Single(executionContext.HtmlAttributes);
             Assert.Equal("someattr", htmlAttribute.Name, StringComparer.Ordinal);
             var htmlContent = Assert.IsAssignableFrom<IHtmlContent>(htmlAttribute.Value);
             Assert.Equal(expectedValue, HtmlContentUtilities.HtmlContentToString(htmlContent), StringComparer.Ordinal);
@@ -1016,7 +1016,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             page.EndAddHtmlAttributeValues(executionContext);
 
             // Assert
-            Assert.Empty(executionContext.HTMLAttributes);
+            Assert.Empty(executionContext.HtmlAttributes);
             var attribute = Assert.Single(executionContext.AllAttributes);
             Assert.Equal("someattr", attribute.Name, StringComparer.Ordinal);
             Assert.Equal(expectedValue, (string)attribute.Value, StringComparer.Ordinal);
@@ -1044,7 +1044,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
             page.EndAddHtmlAttributeValues(executionContext);
 
             // Assert
-            var htmlAttribute = Assert.Single(executionContext.HTMLAttributes);
+            var htmlAttribute = Assert.Single(executionContext.HtmlAttributes);
             Assert.Equal("someattr", htmlAttribute.Name, StringComparer.Ordinal);
             Assert.Equal("someattr", (string)htmlAttribute.Value, StringComparer.Ordinal);
             Assert.False(htmlAttribute.Minimized);
diff --git a/test/Microsoft.AspNetCore.Mvc.Razor.Test/TagHelpers/UrlResolutionTagHelperTest.cs b/test/Microsoft.AspNetCore.Mvc.Razor.Test/TagHelpers/UrlResolutionTagHelperTest.cs
index f7186f7b33..669dab0cd3 100644
--- a/test/Microsoft.AspNetCore.Mvc.Razor.Test/TagHelpers/UrlResolutionTagHelperTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.Razor.Test/TagHelpers/UrlResolutionTagHelperTest.cs
@@ -59,8 +59,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor.TagHelpers
             var tagHelper = new UrlResolutionTagHelper(urlHelperFactory.Object, new HtmlTestEncoder());
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
@@ -115,8 +115,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor.TagHelpers
             var tagHelper = new UrlResolutionTagHelper(urlHelperFactory.Object, new HtmlTestEncoder());
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
@@ -170,8 +170,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor.TagHelpers
             var tagHelper = new UrlResolutionTagHelper(urlHelperFactory.Object, new HtmlTestEncoder());
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
@@ -225,8 +225,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor.TagHelpers
             var tagHelper = new UrlResolutionTagHelper(urlHelperFactory.Object, new HtmlTestEncoder());
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
@@ -255,8 +255,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor.TagHelpers
             var tagHelper = new UrlResolutionTagHelper(urlHelperFactory: null, htmlEncoder: null);
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
@@ -300,8 +300,8 @@ namespace Microsoft.AspNetCore.Mvc.Razor.TagHelpers
             var tagHelper = new UrlResolutionTagHelper(urlHelperFactory.Object, new HtmlTestEncoder());
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/AnchorTagHelperTest.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/AnchorTagHelperTest.cs
index e9f2912a92..18ae391e8f 100644
--- a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/AnchorTagHelperTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/AnchorTagHelperTest.cs
@@ -92,8 +92,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
         {
             // Arrange
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var output = new TagHelperOutput(
@@ -141,8 +141,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
         {
             // Arrange
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var output = new TagHelperOutput(
@@ -225,8 +225,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                                        "'asp-fragment' attribute.";
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
@@ -260,8 +260,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 "'asp-route' must not have an 'asp-action' or 'asp-controller' attribute.";
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/FormTagHelperTest.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/FormTagHelperTest.cs
index 747f4fd751..2a3c1503e3 100644
--- a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/FormTagHelperTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/FormTagHelperTest.cs
@@ -114,8 +114,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var viewContext = CreateViewContext();
             var expectedAttribute = new TagHelperAttribute("method", method.ToString().ToLowerInvariant());
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(new List<IReadOnlyTagHelperAttribute> { expectedAttribute })),
+                allAttributes: new TagHelperAttributeList(new[] { expectedAttribute }),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var output = new TagHelperOutput(
@@ -167,8 +166,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             // Arrange
             var testViewContext = CreateViewContext();
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var expectedAttribute = new TagHelperAttribute("asp-ROUTEE-NotRoute", "something");
@@ -239,8 +238,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             // Arrange
             var viewContext = CreateViewContext();
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var output = new TagHelperOutput(
@@ -290,8 +289,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             // Arrange
             var viewContext = CreateViewContext();
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var output = new TagHelperOutput(
@@ -371,8 +370,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     return Task.FromResult<TagHelperContent>(tagHelperContent);
                 });
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
@@ -419,8 +418,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                                        "'asp-action' or 'asp-controller' or 'asp-route' attribute.";
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
@@ -450,8 +449,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 "'asp-route' must not have an 'asp-action' or 'asp-controller' attribute.";
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/InputTagHelperTest.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/InputTagHelperTest.cs
index f3d35e67e7..58228da9e9 100644
--- a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/InputTagHelperTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/InputTagHelperTest.cs
@@ -87,8 +87,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 "<input name=\"HtmlEncode[[IsACar]]\" type=\"HtmlEncode[[hidden]]\" value=\"HtmlEncode[[false]]\" />";
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var output = new TagHelperOutput(
@@ -193,8 +193,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var expectedTagName = "not-input";
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var originalAttributes = new TagHelperAttributeList
@@ -257,8 +257,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var expectedPostContent = "original post-content";
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var originalAttributes = new TagHelperAttributeList
@@ -343,7 +343,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             };
             if (!string.IsNullOrEmpty(inputTypeName))
             {
-                contextAttributes["type"] = inputTypeName;  // Support restoration of type attribute, if any.
+                contextAttributes.SetAttribute("type", inputTypeName);  // Support restoration of type attribute, if any.
             }
 
             var expectedAttributes = new TagHelperAttributeList
@@ -444,7 +444,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             };
             if (!string.IsNullOrEmpty(inputTypeName))
             {
-                contextAttributes["type"] = inputTypeName;  // Support restoration of type attribute, if any.
+                contextAttributes.SetAttribute("type", inputTypeName);  // Support restoration of type attribute, if any.
             }
 
             var expectedAttributes = new TagHelperAttributeList
@@ -541,7 +541,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             };
             if (!string.IsNullOrEmpty(inputTypeName))
             {
-                contextAttributes["type"] = inputTypeName;  // Support restoration of type attribute, if any.
+                contextAttributes.SetAttribute("type", inputTypeName);  // Support restoration of type attribute, if any.
             }
 
             var expectedAttributes = new TagHelperAttributeList
@@ -651,7 +651,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             };
             if (!string.IsNullOrEmpty(inputTypeName))
             {
-                contextAttributes["type"] = inputTypeName;  // Support restoration of type attribute, if any.
+                contextAttributes.SetAttribute("type", inputTypeName);  // Support restoration of type attribute, if any.
             }
 
             var expectedAttributes = new TagHelperAttributeList
@@ -784,8 +784,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var expectedTagName = "not-input";
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
@@ -868,8 +868,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var expectedTagName = "not-input";
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/Internal/AttributeMatcherTest.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/Internal/AttributeMatcherTest.cs
index 2293b9f48a..870fcc2620 100644
--- a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/Internal/AttributeMatcherTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/Internal/AttributeMatcherTest.cs
@@ -24,8 +24,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers.Internal
             };
             var attributes = new TagHelperAttributeList
             {
-                ["first-attr"] = "value",
-                ["not-in-any-mode"] = "value"
+                new TagHelperAttribute("first-attr", "value"),
+                new TagHelperAttribute("not-in-any-mode", "value")
             };
             var context = MakeTagHelperContext(attributes);
 
@@ -49,9 +49,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers.Internal
             };
             var attributes = new TagHelperAttributeList
             {
-                ["first-attr"] = "value",
-                ["second-attr"] = "value",
-                ["not-in-any-mode"] = "value"
+                new TagHelperAttribute("first-attr", "value"),
+                new TagHelperAttribute("second-attr", "value"),
+                new TagHelperAttribute("not-in-any-mode", "value")
             };
             var context = MakeTagHelperContext(attributes);
 
@@ -77,10 +77,10 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers.Internal
             };
             var attributes = new TagHelperAttributeList
             {
-                ["first-attr"] = "value",
-                ["second-attr"] = "value",
-                ["third-attr"] = "value",
-                ["not-in-any-mode"] = "value"
+                new TagHelperAttribute("first-attr", "value"),
+                new TagHelperAttribute("second-attr", "value"),
+                new TagHelperAttribute("third-attr", "value"),
+                new TagHelperAttribute("not-in-any-mode", "value")
             };
             var context = MakeTagHelperContext(attributes);
 
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/LabelTagHelperTest.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/LabelTagHelperTest.cs
index 8216785fd3..b8f8c9b680 100644
--- a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/LabelTagHelperTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/LabelTagHelperTest.cs
@@ -190,8 +190,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var expectedPostContent = "original post-content";
 
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var htmlAttributes = new TagHelperAttributeList
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/LinkTagHelperTest.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/LinkTagHelperTest.cs
index 372004fb3e..5d55379a80 100644
--- a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/LinkTagHelperTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/LinkTagHelperTest.cs
@@ -200,10 +200,10 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-fallback-href"] = "test.css",
-                            ["asp-fallback-test-class"] = "hidden",
-                            ["asp-fallback-test-property"] = "visibility",
-                            ["asp-fallback-test-value"] = "hidden"
+                            new TagHelperAttribute("asp-fallback-href", "test.css"),
+                            new TagHelperAttribute("asp-fallback-test-class", "hidden"),
+                            new TagHelperAttribute("asp-fallback-test-property", "visibility"),
+                            new TagHelperAttribute("asp-fallback-test-value", "hidden")
                         },
                         tagHelper =>
                         {
@@ -216,10 +216,10 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-fallback-href-include"] = "*.css",
-                            ["asp-fallback-test-class"] = "hidden",
-                            ["asp-fallback-test-property"] = "visibility",
-                            ["asp-fallback-test-value"] = "hidden"
+                            new TagHelperAttribute("asp-fallback-href-include", "*.css"),
+                            new TagHelperAttribute("asp-fallback-test-class", "hidden"),
+                            new TagHelperAttribute("asp-fallback-test-property", "visibility"),
+                            new TagHelperAttribute("asp-fallback-test-value", "hidden")
                         },
                         tagHelper =>
                         {
@@ -233,11 +233,11 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-fallback-href"] = "test.css",
-                            ["asp-fallback-test-class"] = "hidden",
-                            ["asp-fallback-test-property"] = "visibility",
-                            ["asp-fallback-test-value"] = "hidden",
-                            ["asp-append-version"] = "true"
+                            new TagHelperAttribute("asp-fallback-href", "test.css"),
+                            new TagHelperAttribute("asp-fallback-test-class", "hidden"),
+                            new TagHelperAttribute("asp-fallback-test-property", "visibility"),
+                            new TagHelperAttribute("asp-fallback-test-value", "hidden"),
+                            new TagHelperAttribute("asp-append-version", "true")
                         },
                         tagHelper =>
                         {
@@ -251,11 +251,11 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-fallback-href-include"] = "*.css",
-                            ["asp-fallback-test-class"] = "hidden",
-                            ["asp-fallback-test-property"] = "visibility",
-                            ["asp-fallback-test-value"] = "hidden",
-                            ["asp-append-version"] = "true"
+                            new TagHelperAttribute("asp-fallback-href-include", "*.css"),
+                            new TagHelperAttribute("asp-fallback-test-class", "hidden"),
+                            new TagHelperAttribute("asp-fallback-test-property", "visibility"),
+                            new TagHelperAttribute("asp-fallback-test-value", "hidden"),
+                            new TagHelperAttribute("asp-append-version", "true")
                         },
                         tagHelper =>
                         {
@@ -318,7 +318,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-href-include"] = "*.css"
+                            new TagHelperAttribute("asp-href-include", "*.css")
                         },
                         tagHelper =>
                         {
@@ -328,8 +328,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-href-include"] = "*.css",
-                            ["asp-href-exclude"] = "*.min.css"
+                            new TagHelperAttribute("asp-href-include", "*.css"),
+                            new TagHelperAttribute("asp-href-exclude", "*.min.css")
                         },
                         tagHelper =>
                         {
@@ -340,8 +340,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-href-include"] = "*.css",
-                            ["asp-append-version"] = "true"
+                            new TagHelperAttribute("asp-href-include", "*.css"),
+                            new TagHelperAttribute("asp-append-version", "true")
                         },
                         tagHelper =>
                         {
@@ -352,9 +352,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-href-include"] = "*.css",
-                            ["asp-href-exclude"] = "*.min.css",
-                            ["asp-append-version"] = "true"
+                            new TagHelperAttribute("asp-href-include", "*.css"),
+                            new TagHelperAttribute("asp-href-exclude", "*.min.css"),
+                            new TagHelperAttribute("asp-append-version", "true")
                         },
                         tagHelper =>
                         {
@@ -463,8 +463,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            // This is commented out on purpose: ["asp-href-include"] = "*.css",
-                            ["asp-href-exclude"] = "*.min.css"
+                            // This is commented out on purpose: new TagHelperAttribute("asp-href-include", "*.css"),
+                            // Note asp-href-include attribute isn't included.
+                            new TagHelperAttribute("asp-href-exclude", "*.min.css")
                         },
                         tagHelper =>
                         {
@@ -475,10 +476,11 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            // This is commented out on purpose: ["asp-fallback-href"] = "test.css",
-                            ["asp-fallback-test-class"] = "hidden",
-                            ["asp-fallback-test-property"] = "visibility",
-                            ["asp-fallback-test-value"] = "hidden"
+                            // This is commented out on purpose: new TagHelperAttribute("asp-fallback-href", "test.css"),
+                            // Note asp-href-include attribute isn't included.
+                            new TagHelperAttribute("asp-fallback-test-class", "hidden"),
+                            new TagHelperAttribute("asp-fallback-test-property", "visibility"),
+                            new TagHelperAttribute("asp-fallback-test-value", "hidden")
                         },
                         tagHelper =>
                         {
@@ -491,10 +493,11 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-fallback-href"] = "test.css",
-                            ["asp-fallback-test-class"] = "hidden",
-                            // This is commented out on purpose: ["asp-fallback-test-property"] = "visibility",
-                            ["asp-fallback-test-value"] = "hidden"
+                            new TagHelperAttribute("asp-fallback-href", "test.css"),
+                            new TagHelperAttribute("asp-fallback-test-class", "hidden"),
+                            // This is commented out on purpose: new TagHelperAttribute("asp-fallback-test-property", "visibility"),
+                            // Note asp-href-include attribute isn't included.
+                            new TagHelperAttribute("asp-fallback-test-value", "hidden")
                         },
                         tagHelper =>
                         {
@@ -507,11 +510,11 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            // This is commented out on purpose: ["asp-fallback-href-include"] = "test.css",
-                            ["asp-fallback-href-exclude"] = "**/*.min.css",
-                            ["asp-fallback-test-class"] = "hidden",
-                            ["asp-fallback-test-property"] = "visibility",
-                            ["asp-fallback-test-value"] = "hidden"
+                            // This is commented out on purpose: new TagHelperAttribute("asp-fallback-href-include", "test.css"),
+                            new TagHelperAttribute("asp-fallback-href-exclude", "**/*.min.css"),
+                            new TagHelperAttribute("asp-fallback-test-class", "hidden"),
+                            new TagHelperAttribute("asp-fallback-test-property", "visibility"),
+                            new TagHelperAttribute("asp-fallback-test-value", "hidden")
                         },
                         tagHelper =>
                         {
@@ -641,13 +644,13 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var context = MakeTagHelperContext(
                 attributes: new TagHelperAttributeList
                 {
-                    ["rel"] = "stylesheet",
-                    ["href"] = "/css/site.css",
-                    ["asp-href-include"] = "**/*.css"
+                    new TagHelperAttribute("rel", "stylesheet"),
+                    new TagHelperAttribute("href", "/css/site.css"),
+                    new TagHelperAttribute("asp-href-include", "**/*.css")
                 });
             var output = MakeTagHelperOutput("link", attributes: new TagHelperAttributeList
             {
-                ["rel"] = "stylesheet",
+                new TagHelperAttribute("rel", "stylesheet"),
             });
             var hostingEnvironment = MakeHostingEnvironment();
             var viewContext = MakeViewContext();
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/RenderAtEndOfFormTagHelperTest.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/RenderAtEndOfFormTagHelperTest.cs
index 8abe6c1a8d..17093ac74c 100644
--- a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/RenderAtEndOfFormTagHelperTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/RenderAtEndOfFormTagHelperTest.cs
@@ -63,7 +63,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 });
 
             var tagHelperContext = new TagHelperContext(
-                Enumerable.Empty<IReadOnlyTagHelperAttribute>(),
+                Enumerable.Empty<TagHelperAttribute>(),
                 new Dictionary<object, object>(),
                 "someId");
 
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/ScriptTagHelperTest.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/ScriptTagHelperTest.cs
index ebcc42c29d..95286d9a76 100644
--- a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/ScriptTagHelperTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/ScriptTagHelperTest.cs
@@ -102,10 +102,10 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 outputAttributes.Concat(
                     new TagHelperAttributeList
                     {
-                        ["data-extra"] = "something",
-                        ["src"] = "/blank.js",
-                        ["asp-fallback-src"] = "http://www.example.com/blank.js",
-                        ["asp-fallback-test"] = "isavailable()",
+                        new TagHelperAttribute("data-extra", "something"),
+                        new TagHelperAttribute("src", "/blank.js"),
+                        new TagHelperAttribute("asp-fallback-src", "http://www.example.com/blank.js"),
+                        new TagHelperAttribute("asp-fallback-test", "isavailable()"),
                     }));
             var tagHelperContext = MakeTagHelperContext(allAttributes);
             var viewContext = MakeViewContext();
@@ -149,8 +149,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-fallback-src"] = "test.js",
-                            ["asp-fallback-test"] = "isavailable()"
+                            new TagHelperAttribute("asp-fallback-src", "test.js"),
+                            new TagHelperAttribute("asp-fallback-test", "isavailable()")
                         },
                         tagHelper =>
                         {
@@ -161,8 +161,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-fallback-src-include"] = "*.js",
-                            ["asp-fallback-test"] = "isavailable()"
+                            new TagHelperAttribute("asp-fallback-src-include", "*.js"),
+                            new TagHelperAttribute("asp-fallback-test", "isavailable()")
                         },
                         tagHelper =>
                         {
@@ -173,9 +173,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-fallback-src"] = "test.js",
-                            ["asp-fallback-src-include"] = "*.js",
-                            ["asp-fallback-test"] = "isavailable()"
+                            new TagHelperAttribute("asp-fallback-src", "test.js"),
+                            new TagHelperAttribute("asp-fallback-src-include", "*.js"),
+                            new TagHelperAttribute("asp-fallback-test", "isavailable()")
                         },
                         tagHelper =>
                         {
@@ -187,9 +187,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-fallback-src-include"] = "*.js",
-                            ["asp-fallback-src-exclude"] = "*.min.js",
-                            ["asp-fallback-test"] = "isavailable()"
+                            new TagHelperAttribute("asp-fallback-src-include", "*.js"),
+                            new TagHelperAttribute("asp-fallback-src-exclude", "*.min.js"),
+                            new TagHelperAttribute("asp-fallback-test", "isavailable()")
                         },
                         tagHelper =>
                         {
@@ -202,9 +202,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-fallback-src"] = "test.js",
-                            ["asp-fallback-test"] = "isavailable()",
-                            ["asp-append-version"] = "true"
+                            new TagHelperAttribute("asp-fallback-src", "test.js"),
+                            new TagHelperAttribute("asp-fallback-test", "isavailable()"),
+                            new TagHelperAttribute("asp-append-version", "true")
                         },
                         tagHelper =>
                         {
@@ -216,9 +216,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-fallback-src-include"] = "*.js",
-                            ["asp-fallback-test"] = "isavailable()",
-                            ["asp-append-version"] = "true"
+                            new TagHelperAttribute("asp-fallback-src-include", "*.js"),
+                            new TagHelperAttribute("asp-fallback-test", "isavailable()"),
+                            new TagHelperAttribute("asp-append-version", "true")
                         },
                         tagHelper =>
                         {
@@ -230,10 +230,10 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-fallback-src"] = "test.js",
-                            ["asp-fallback-src-include"] = "*.js",
-                            ["asp-fallback-test"] = "isavailable()",
-                            ["asp-append-version"] = "true"
+                            new TagHelperAttribute("asp-fallback-src", "test.js"),
+                            new TagHelperAttribute("asp-fallback-src-include", "*.js"),
+                            new TagHelperAttribute("asp-fallback-test", "isavailable()"),
+                            new TagHelperAttribute("asp-append-version", "true")
                         },
                         tagHelper =>
                         {
@@ -246,10 +246,10 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-fallback-src-include"] = "*.js",
-                            ["asp-fallback-src-exclude"] = "*.min.js",
-                            ["asp-fallback-test"] = "isavailable()",
-                            ["asp-append-version"] = "true"
+                            new TagHelperAttribute("asp-fallback-src-include", "*.js"),
+                            new TagHelperAttribute("asp-fallback-src-exclude", "*.min.js"),
+                            new TagHelperAttribute("asp-fallback-test", "isavailable()"),
+                            new TagHelperAttribute("asp-append-version", "true")
                         },
                         tagHelper =>
                         {
@@ -311,7 +311,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-src-include"] = "*.js"
+                            new TagHelperAttribute("asp-src-include", "*.js")
                         },
                         tagHelper =>
                         {
@@ -321,8 +321,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-src-include"] = "*.js",
-                            ["asp-src-exclude"] = "*.min.js"
+                            new TagHelperAttribute("asp-src-include", "*.js"),
+                            new TagHelperAttribute("asp-src-exclude", "*.min.js")
                         },
                         tagHelper =>
                         {
@@ -333,8 +333,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-src-include"] = "*.js",
-                            ["asp-append-version"] = "true"
+                            new TagHelperAttribute("asp-src-include", "*.js"),
+                            new TagHelperAttribute("asp-append-version", "true")
                         },
                         tagHelper =>
                         {
@@ -345,9 +345,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-src-include"] = "*.js",
-                            ["asp-src-exclude"] = "*.min.js",
-                            ["asp-append-version"] = "true"
+                            new TagHelperAttribute("asp-src-include", "*.js"),
+                            new TagHelperAttribute("asp-src-exclude", "*.min.js"),
+                            new TagHelperAttribute("asp-append-version", "true")
                         },
                         tagHelper =>
                         {
@@ -408,8 +408,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            // This is commented out on purpose: ["asp-src-include"] = "*.js",
-                            ["asp-src-exclude"] = "*.min.js"
+                            // This is commented out on purpose: new TagHelperAttribute("asp-src-include", "*.js"),
+                            // Note asp-src-include attribute isn't included.
+                            new TagHelperAttribute("asp-src-exclude", "*.min.js")
                         },
                         tagHelper =>
                         {
@@ -420,8 +421,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            // This is commented out on purpose: ["asp-fallback-src"] = "test.js",
-                            ["asp-fallback-test"] = "isavailable()",
+                            // This is commented out on purpose: new TagHelperAttribute("asp-fallback-src", "test.js"),
+                            // Note asp-src-include attribute isn't included.
+                            new TagHelperAttribute("asp-fallback-test", "isavailable()"),
                         },
                         tagHelper =>
                         {
@@ -432,8 +434,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            ["asp-fallback-src"] = "test.js",
-                            // This is commented out on purpose: ["asp-fallback-test"] = "isavailable()"
+                            new TagHelperAttribute("asp-fallback-src", "test.js"),
+                            // This is commented out on purpose: new TagHelperAttribute("asp-fallback-test", "isavailable()")
+                            // Note asp-src-include attribute isn't included.
                         },
                         tagHelper =>
                         {
@@ -444,9 +447,10 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     {
                         new TagHelperAttributeList
                         {
-                            // This is commented out on purpose: ["asp-fallback-src-include"] = "test.js",
-                            ["asp-fallback-src-exclude"] = "**/*.min.js",
-                            ["asp-fallback-test"] = "isavailable()",
+                            // This is commented out on purpose: new TagHelperAttribute("asp-fallback-src-include", "test.js"),
+                            // Note asp-src-include attribute isn't included.
+                            new TagHelperAttribute("asp-fallback-src-exclude", "**/*.min.js"),
+                            new TagHelperAttribute("asp-fallback-test", "isavailable()"),
                         },
                         tagHelper =>
                         {
@@ -528,11 +532,11 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var tagHelperContext = MakeTagHelperContext(
                 attributes: new TagHelperAttributeList
                 {
-                    ["data-extra"] = "something",
-                    ["src"] = "/blank.js",
-                    ["data-more"] = "else",
-                    ["asp-fallback-src"] = "http://www.example.com/blank.js",
-                    ["asp-fallback-test"] = "isavailable()",
+                    new TagHelperAttribute("data-extra", "something"),
+                    new TagHelperAttribute("src", "/blank.js"),
+                    new TagHelperAttribute("data-more", "else"),
+                    new TagHelperAttribute("asp-fallback-src", "http://www.example.com/blank.js"),
+                    new TagHelperAttribute("asp-fallback-test", "isavailable()"),
                 });
 
             var viewContext = MakeViewContext();
@@ -540,8 +544,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var output = MakeTagHelperOutput("src",
                 attributes: new TagHelperAttributeList
                 {
-                    ["data-extra"] = "something",
-                    ["data-more"] = "else",
+                    new TagHelperAttribute("data-extra", "something"),
+                    new TagHelperAttribute("data-more", "else"),
                 });
 
             var hostingEnvironment = MakeHostingEnvironment();
@@ -575,8 +579,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var context = MakeTagHelperContext(
                 attributes: new TagHelperAttributeList
                 {
-                    ["src"] = "/js/site.js",
-                    ["asp-src-include"] = "**/*.js"
+                    new TagHelperAttribute("src", "/js/site.js"),
+                    new TagHelperAttribute("asp-src-include", "**/*.js")
                 });
             var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());
             var hostingEnvironment = MakeHostingEnvironment();
@@ -617,8 +621,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var context = MakeTagHelperContext(
                 attributes: new TagHelperAttributeList
                 {
-                    ["src"] = "/js/site.js",
-                    ["asp-src-include"] = "**/*.js"
+                    new TagHelperAttribute("src", "/js/site.js"),
+                    new TagHelperAttribute("asp-src-include", "**/*.js")
                 });
             var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());
             var hostingEnvironment = MakeHostingEnvironment();
@@ -659,8 +663,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var context = MakeTagHelperContext(
                 attributes: new TagHelperAttributeList
                 {
-                    ["src"] = "/js/site.js",
-                    ["asp-append-version"] = "true"
+                    new TagHelperAttribute("src", "/js/site.js"),
+                    new TagHelperAttribute("asp-append-version", "true")
                 });
             var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());
 
@@ -694,8 +698,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var context = MakeTagHelperContext(
                 attributes: new TagHelperAttributeList
                 {
-                    ["src"] = "/bar/js/site.js",
-                    ["asp-append-version"] = "true"
+                    new TagHelperAttribute("src", "/bar/js/site.js"),
+                    new TagHelperAttribute("asp-append-version", "true")
                 });
             var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());
             var hostingEnvironment = MakeHostingEnvironment();
@@ -728,10 +732,10 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var context = MakeTagHelperContext(
                 attributes: new TagHelperAttributeList
                 {
-                    ["src"] = "/js/site.js",
-                    ["asp-fallback-src-include"] = "fallback.js",
-                    ["asp-fallback-test"] = "isavailable()",
-                    ["asp-append-version"] = "true"
+                    new TagHelperAttribute("src", "/js/site.js"),
+                    new TagHelperAttribute("asp-fallback-src-include", "fallback.js"),
+                    new TagHelperAttribute("asp-fallback-test", "isavailable()"),
+                    new TagHelperAttribute("asp-append-version", "true")
                 });
             var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());
             var hostingEnvironment = MakeHostingEnvironment();
@@ -769,9 +773,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var context = MakeTagHelperContext(
                 attributes: new TagHelperAttributeList
                 {
-                    ["src"] = "/js/site.js",
-                    ["asp-src-include"] = "*.js",
-                    ["asp-append-version"] = "true"
+                    new TagHelperAttribute("src", "/js/site.js"),
+                    new TagHelperAttribute("asp-src-include", "*.js"),
+                    new TagHelperAttribute("asp-append-version", "true")
                 });
             var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());
             var hostingEnvironment = MakeHostingEnvironment();
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/SelectTagHelperTest.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/SelectTagHelperTest.cs
index 7652bbc23d..ca79168126 100644
--- a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/SelectTagHelperTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/SelectTagHelperTest.cs
@@ -201,8 +201,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var modelExpression = new ModelExpression(nameAndId.Name, modelExplorer);
 
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var output = new TagHelperOutput(
@@ -290,8 +290,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var modelExpression = new ModelExpression(nameAndId.Name, modelExplorer);
 
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var output = new TagHelperOutput(
@@ -394,8 +394,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var modelExpression = new ModelExpression(name: string.Empty, modelExplorer: modelExplorer);
 
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var output = new TagHelperOutput(
@@ -557,8 +557,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             bool allowMultiple)
         {
             // Arrange
-            var contextAttributes = new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>());
+            var contextAttributes = new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>());
             var originalAttributes = new TagHelperAttributeList();
             var propertyName = "Property1";
             var tagName = "select";
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/TagHelperOutputExtensionsTest.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/TagHelperOutputExtensionsTest.cs
index 1b903005db..0784d3f29c 100644
--- a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/TagHelperOutputExtensionsTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/TagHelperOutputExtensionsTest.cs
@@ -960,7 +960,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             Assert.Equal(expectedBuilderAttribute.Value, attribute.Value);
         }
 
-        private class CaseSensitiveTagHelperAttributeComparer : IEqualityComparer<IReadOnlyTagHelperAttribute>
+        private class CaseSensitiveTagHelperAttributeComparer : IEqualityComparer<TagHelperAttribute>
         {
             public readonly static CaseSensitiveTagHelperAttributeComparer Default =
                 new CaseSensitiveTagHelperAttributeComparer();
@@ -969,7 +969,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             {
             }
 
-            public bool Equals(IReadOnlyTagHelperAttribute attributeX, IReadOnlyTagHelperAttribute attributeY)
+            public bool Equals(TagHelperAttribute attributeX, TagHelperAttribute attributeY)
             {
                 if (attributeX == attributeY)
                 {
@@ -983,7 +983,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     (attributeX.Minimized || Equals(attributeX.Value, attributeY.Value));
             }
 
-            public int GetHashCode(IReadOnlyTagHelperAttribute attribute)
+            public int GetHashCode(TagHelperAttribute attribute)
             {
                 return attribute.GetHashCode();
             }
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/TextAreaTagHelperTest.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/TextAreaTagHelperTest.cs
index 24d1375a0b..3174573145 100644
--- a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/TextAreaTagHelperTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/TextAreaTagHelperTest.cs
@@ -124,8 +124,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             };
 
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var htmlAttributes = new TagHelperAttributeList
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/ValidationMessageTagHelperTest.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/ValidationMessageTagHelperTest.cs
index d4e6fcfe93..d032ac1b35 100644
--- a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/ValidationMessageTagHelperTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/ValidationMessageTagHelperTest.cs
@@ -114,8 +114,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var expectedContent = "original content";
             var expectedPostContent = "original post-content";
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var output = new TagHelperOutput(
@@ -184,8 +184,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             output.Content.AppendHtml(outputContent);
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
@@ -243,8 +243,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 });
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
@@ -283,8 +283,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             output.PostContent.SetContent(expectedPostContent);
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/ValidationSummaryTagHelperTest.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/ValidationSummaryTagHelperTest.cs
index 760e2c6fac..1432219455 100644
--- a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/ValidationSummaryTagHelperTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/ValidationSummaryTagHelperTest.cs
@@ -39,8 +39,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var expectedPreContent = "original pre-content";
             var expectedContent = "original content";
             var tagHelperContext = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
             var output = new TagHelperOutput(
@@ -120,8 +120,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             validationSummaryTagHelper.ViewContext = expectedViewContext;
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
@@ -176,8 +176,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             validationSummaryTagHelper.ViewContext = viewContext;
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
@@ -225,8 +225,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             validationSummaryTagHelper.ViewContext = viewContext;
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
@@ -281,8 +281,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             validationSummaryTagHelper.ViewContext = viewContext;
 
             var context = new TagHelperContext(
-                allAttributes: new ReadOnlyTagHelperAttributeList<IReadOnlyTagHelperAttribute>(
-                    Enumerable.Empty<IReadOnlyTagHelperAttribute>()),
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
                 items: new Dictionary<object, object>(),
                 uniqueId: "test");
 
diff --git a/test/WebSites/TagHelpersWebSite/TagHelpers/ATagHelper.cs b/test/WebSites/TagHelpersWebSite/TagHelpers/ATagHelper.cs
index 7129d812bd..2326ee9ca6 100644
--- a/test/WebSites/TagHelpersWebSite/TagHelpers/ATagHelper.cs
+++ b/test/WebSites/TagHelpersWebSite/TagHelpers/ATagHelper.cs
@@ -40,7 +40,7 @@ namespace TagHelpersWebSite.TagHelpers
                 output.Attributes.Clear();
 
                 var urlHelper = UrlHelperFactory.GetUrlHelper(ViewContext);
-                output.Attributes["href"] = urlHelper.Action(Action, Controller, methodParameters);
+                output.Attributes.SetAttribute("href", urlHelper.Action(Action, Controller, methodParameters));
 
                 output.PreContent.SetContent("My ");
             }
diff --git a/test/WebSites/TagHelpersWebSite/TagHelpers/AddProcessedAttributeTagHelper.cs b/test/WebSites/TagHelpersWebSite/TagHelpers/AddProcessedAttributeTagHelper.cs
index 8db1ba7424..88e62e0305 100644
--- a/test/WebSites/TagHelpersWebSite/TagHelpers/AddProcessedAttributeTagHelper.cs
+++ b/test/WebSites/TagHelpersWebSite/TagHelpers/AddProcessedAttributeTagHelper.cs
@@ -10,7 +10,7 @@ namespace TagHelpersWebSite.TagHelpers
     {
         public override void Process(TagHelperContext context, TagHelperOutput output)
         {
-            output.Attributes.Add(new TagHelperAttribute { Name = "processed", Minimized = true });
+            output.Attributes.Add(new TagHelperAttribute("processed"));
         }
     }
 }
diff --git a/test/WebSites/TagHelpersWebSite/TagHelpers/PrettyTagHelper.cs b/test/WebSites/TagHelpersWebSite/TagHelpers/PrettyTagHelper.cs
index cd8ee00942..267e410199 100644
--- a/test/WebSites/TagHelpersWebSite/TagHelpers/PrettyTagHelper.cs
+++ b/test/WebSites/TagHelpersWebSite/TagHelpers/PrettyTagHelper.cs
@@ -53,7 +53,7 @@ namespace TagHelpersWebSite.TagHelpers
                     style += ";";
                 }
 
-                output.Attributes["style"] = style + prettyStyle;
+                output.Attributes.SetAttribute("style", style + prettyStyle);
             }
         }
     }