diff --git a/src/Html.Abstractions/src/HtmlContentBuilderExtensions.cs b/src/Html.Abstractions/src/HtmlContentBuilderExtensions.cs
index 53be2049e3d9..202893e162cf 100644
--- a/src/Html.Abstractions/src/HtmlContentBuilderExtensions.cs
+++ b/src/Html.Abstractions/src/HtmlContentBuilderExtensions.cs
@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics.CodeAnalysis;
+
 namespace Microsoft.AspNetCore.Html;
 
 /// <summary>
@@ -24,7 +26,7 @@ public static class HtmlContentBuilderExtensions
     /// <returns>A reference to this instance after the append operation has completed.</returns>
     public static IHtmlContentBuilder AppendFormat(
         this IHtmlContentBuilder builder,
-        string format,
+        [StringSyntax(StringSyntaxAttribute.CompositeFormat)] string format,
         params object?[] args)
     {
         if (builder == null)
@@ -64,7 +66,7 @@ public static IHtmlContentBuilder AppendFormat(
     public static IHtmlContentBuilder AppendFormat(
         this IHtmlContentBuilder builder,
         IFormatProvider formatProvider,
-        string format,
+        [StringSyntax(StringSyntaxAttribute.CompositeFormat)] string format,
         params object?[] args)
     {
         if (builder == null)
diff --git a/src/Html.Abstractions/src/HtmlFormattableString.cs b/src/Html.Abstractions/src/HtmlFormattableString.cs
index 5bbc6219966a..0de6d6482b6a 100644
--- a/src/Html.Abstractions/src/HtmlFormattableString.cs
+++ b/src/Html.Abstractions/src/HtmlFormattableString.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Text.Encodings.Web;
 
@@ -25,7 +26,7 @@ public class HtmlFormattableString : IHtmlContent
     /// </summary>
     /// <param name="format">A composite format string.</param>
     /// <param name="args">An array that contains objects to format.</param>
-    public HtmlFormattableString(string format, params object?[] args)
+    public HtmlFormattableString([StringSyntax(StringSyntaxAttribute.CompositeFormat)] string format, params object?[] args)
         : this(formatProvider: null, format: format, args: args)
     {
     }
@@ -37,7 +38,10 @@ public HtmlFormattableString(string format, params object?[] args)
     /// <param name="formatProvider">An object that provides culture-specific formatting information.</param>
     /// <param name="format">A composite format string.</param>
     /// <param name="args">An array that contains objects to format.</param>
-    public HtmlFormattableString(IFormatProvider? formatProvider, string format, params object?[] args)
+    public HtmlFormattableString(
+        IFormatProvider? formatProvider,
+        [StringSyntax(StringSyntaxAttribute.CompositeFormat)] string format,
+        params object?[] args)
     {
         if (format == null)
         {
diff --git a/src/Razor/Razor/src/TagHelpers/TagHelperContent.cs b/src/Razor/Razor/src/TagHelpers/TagHelperContent.cs
index a8747b2bb101..b528e63edf25 100644
--- a/src/Razor/Razor/src/TagHelpers/TagHelperContent.cs
+++ b/src/Razor/Razor/src/TagHelpers/TagHelperContent.cs
@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Encodings.Web;
 using Microsoft.AspNetCore.Html;
 
@@ -92,7 +93,7 @@ public TagHelperContent SetHtmlContent(string encoded)
     /// </param>
     /// <param name="args">The object array to format.</param>
     /// <returns>A reference to this instance after the append operation has completed.</returns>
-    public TagHelperContent AppendFormat(string format, params object[] args)
+    public TagHelperContent AppendFormat([StringSyntax(StringSyntaxAttribute.CompositeFormat)] string format, params object[] args)
     {
         HtmlContentBuilderExtensions.AppendFormat(this, null, format, args);
         return this;
@@ -109,7 +110,10 @@ public TagHelperContent AppendFormat(string format, params object[] args)
     /// </param>
     /// <param name="args">The object array to format.</param>
     /// <returns>A reference to this instance after the append operation has completed.</returns>
-    public TagHelperContent AppendFormat(IFormatProvider provider, string format, params object[] args)
+    public TagHelperContent AppendFormat(
+        IFormatProvider provider,
+        [StringSyntax(StringSyntaxAttribute.CompositeFormat)] string format,
+        params object[] args)
     {
         HtmlContentBuilderExtensions.AppendFormat(this, provider, format, args);
         return this;