diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7e073e41..0ae2a913 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 Released on Monday, November 2 2026.
 
 - Improved the parser with a nesting limit for values and conditions (`MaxValueNestingDepth`) (#260) @michaelthyregod
+- Improved indexing rules with deeply nested `:is()`, `:where()`, or `:matches()` (#261) @michaelthyregod
 
 # 1.3.1
 
diff --git a/src/AngleSharp.Css.Tests/Styling/StyleRuleIndexTests.cs b/src/AngleSharp.Css.Tests/Styling/StyleRuleIndexTests.cs
index fcd55a33..a74b71d4 100644
--- a/src/AngleSharp.Css.Tests/Styling/StyleRuleIndexTests.cs
+++ b/src/AngleSharp.Css.Tests/Styling/StyleRuleIndexTests.cs
@@ -72,6 +72,7 @@ public async Task GroupedAndUngroupedRulesKeepTheCascadeOrder(string css)
         [TestCase(":where(.x .a, div) { color: green }", "<div id=t>A</div>")]
         [TestCase(":matches(.x, .b) { color: green }", "<div id=t class=b>A</div>")]
         [TestCase(":is(:is(.x, .b)) { color: green }", "<div id=t class=b>A</div>")]
+        [TestCase(":is(:is(:is(:is(:is(:is(:is(:is(:is(.b))))))))) { color: green }", "<div id=t class=b>A</div>")]
         [TestCase(":is(.x, *) { color: green }", "<div id=t>A</div>")]
         [TestCase(":is(.x, :not(.y)) { color: green }", "<div id=t>A</div>")]
         [TestCase(".parent { :is(&) { color: green } }", "<p id=t class=parent>A</p>")]
@@ -89,6 +90,7 @@ public async Task RulesAreFoundViaTheirSubject(string css, string body)
         [TestCase(".a .b { color: red }", "<div class=b><p id=t class=a>A</p></div>")]
         [TestCase("[data-x] { color: red }", "<p id=t data-y>A</p>")]
         [TestCase(":is(.x .a, .b) { color: red }", "<p id=t class=a>A</p>")]
+        [TestCase(":is(:is(:is(:is(:is(:is(:is(:is(:is(.x))))))))) { color: red }", "<p id=t class=a>A</p>")]
         public async Task RulesForOtherSubjectsDoNotApply(string css, string body)
         {
             var color = await ColorOfAsync(css, body);
@@ -102,6 +104,9 @@ public async Task RulesForOtherSubjectsDoNotApply(string css, string body)
         [TestCase(":is(.x, .a) { color: red }", 1)]
         [TestCase(":where(.x .y, #z) { color: red }", 0)]
         [TestCase(":is(.x, :not(.y)) { color: red }", 1)]
+        // the index looks into 8 levels of :is(); a deeper rule stays ungrouped, i.e., a candidate
+        [TestCase(":is(:is(:is(:is(:is(:is(:is(:is(.x)))))))) { color: red }", 0)]
+        [TestCase(":is(:is(:is(:is(:is(:is(:is(:is(:is(.x))))))))) { color: red }", 1)]
         [TestCase("! { color: red }", 0)]
         public void OnlyRulesThatMayMatchAreCandidates(string css, int count)
         {
diff --git a/src/AngleSharp.Css/Dom/Internal/StyleRuleIndex.cs b/src/AngleSharp.Css/Dom/Internal/StyleRuleIndex.cs
index b1c59514..6e860eb6 100644
--- a/src/AngleSharp.Css/Dom/Internal/StyleRuleIndex.cs
+++ b/src/AngleSharp.Css/Dom/Internal/StyleRuleIndex.cs
@@ -355,17 +355,27 @@ private sealed class SubjectKeyVisitor : ISelectorVisitor
                 PseudoClassNames.Matches,
             };
 
+            // How many :is() levels are looked into. Each one parses its arguments again, so a
+            // deeply nested selector would cost quadratic time (and stack) to index; real ones
+            // nest a level or two. Deeper, the rule just stays ungrouped, which is always correct.
+            private const Int32 MaxArgumentDepth = 8;
+
             private readonly ISelector _selector;
             private readonly ICssSelectorParser? _parser;
+            private readonly Int32 _depth;
             private SubjectKey[]? _keys;
 
-            private SubjectKeyVisitor(ISelector selector, ICssSelectorParser? parser)
+            private SubjectKeyVisitor(ISelector selector, ICssSelectorParser? parser, Int32 depth)
             {
                 _selector = selector;
                 _parser = parser;
+                _depth = depth;
             }
 
-            public static SubjectKey[]? GetKeys(ISelector? selector, ICssSelectorParser? parser)
+            public static SubjectKey[]? GetKeys(ISelector? selector, ICssSelectorParser? parser) =>
+                GetKeys(selector, parser, 0);
+
+            private static SubjectKey[]? GetKeys(ISelector? selector, ICssSelectorParser? parser, Int32 depth)
             {
                 // a forced pseudo-class matches independent of its own selector
                 if (selector is null || selector is ForcingPseudoClassSelector)
@@ -373,7 +383,7 @@ private SubjectKeyVisitor(ISelector selector, ICssSelectorParser? parser)
                     return null;
                 }
 
-                var visitor = new SubjectKeyVisitor(selector, parser);
+                var visitor = new SubjectKeyVisitor(selector, parser, depth);
                 selector.Accept(visitor);
                 return visitor._keys;
             }
@@ -409,7 +419,7 @@ void ISelectorVisitor.PseudoClass(String name)
             {
                 // :is(.a .b, .c) only exposes its text, but an element matching it matches one
                 // of the arguments, i.e., it requires one of their keys - just as a list does
-                var arguments = GetArguments(name);
+                var arguments = _depth < MaxArgumentDepth ? GetArguments(name) : null;
 
                 if (arguments is not null && _parser is not null)
                 {
@@ -419,7 +429,7 @@ void ISelectorVisitor.PseudoClass(String name)
                     // are valid only where the rule is (e.g., nested); then the keys are incomplete
                     if (selector is not null && selector.Text.Equals(arguments, StringComparison.Ordinal))
                     {
-                        _keys = GetKeys(selector, _parser);
+                        _keys = GetKeys(selector, _parser, _depth + 1);
                     }
                 }
             }
@@ -434,7 +444,7 @@ void ISelectorVisitor.List(IEnumerable<ISelector> selectors)
 
                 foreach (var selector in selectors)
                 {
-                    var alternative = GetKeys(selector, _parser);
+                    var alternative = GetKeys(selector, _parser, _depth);
 
                     if (alternative is null)
                     {
@@ -449,7 +459,7 @@ void ISelectorVisitor.List(IEnumerable<ISelector> selectors)
 
             void ISelectorVisitor.Combinator(IEnumerable<ISelector> selectors, IEnumerable<String> symbols)
             {
-                _keys = GetKeys(selectors.LastOrDefault(), _parser);
+                _keys = GetKeys(selectors.LastOrDefault(), _parser, _depth);
             }
 
             void ISelectorVisitor.Many(IEnumerable<ISelector> selectors)
@@ -457,7 +467,7 @@ void ISelectorVisitor.Many(IEnumerable<ISelector> selectors)
                 // all parts of a compound must match, so any key of a part is required
                 foreach (var selector in selectors)
                 {
-                    var keys = GetKeys(selector, _parser);
+                    var keys = GetKeys(selector, _parser, _depth);
 
                     if (keys is not null && (_keys is null || IsBetter(keys, _keys)))
                     {
diff --git a/src/AngleSharp.Performance.Css/DeepSelectorBenchmarks.cs b/src/AngleSharp.Performance.Css/DeepSelectorBenchmarks.cs
new file mode 100644
index 00000000..b10f9d2c
--- /dev/null
+++ b/src/AngleSharp.Performance.Css/DeepSelectorBenchmarks.cs
@@ -0,0 +1,44 @@
+namespace AngleSharp.Performance.Css
+{
+    using AngleSharp;
+    using AngleSharp.Css.Dom;
+    using AngleSharp.Dom;
+    using AngleSharp.Html.Parser;
+    using BenchmarkDotNet.Attributes;
+    using System;
+    using System.Text;
+
+    /// <summary>
+    /// Computing a style with a rule whose selector nests :is(): the cascade indexes the rule by
+    /// looking into the arguments. The depths stay within the 75 levels AngleSharp allows from
+    /// 1.9.1 on (Firefox's limit).
+    /// </summary>
+    [MemoryDiagnoser]
+    public class DeepSelectorBenchmarks
+    {
+        private IHtmlParser _parser = null!;
+        private String _source = null!;
+
+        [Params(10, 50, 75)]
+        public Int32 Depth { get; set; }
+
+        [GlobalSetup]
+        public void Setup()
+        {
+            var context = BrowsingContext.New(Configuration.Default.WithCss());
+            _parser = context.GetService<IHtmlParser>()!;
+            _source = "<style>" + Repeat(":is(", Depth) + ".x" + Repeat(")", Depth) + "{color:red}</style><div class=x>x</div>";
+        }
+
+        [Benchmark]
+        public String ComputedStyle()
+        {
+            // a fresh document, so the rule is indexed again (the index is kept with the rule)
+            var document = _parser.ParseDocument(_source);
+            return document.DefaultView!.GetComputedStyle(document.QuerySelector("div")!).GetColor();
+        }
+
+        private static String Repeat(String value, Int32 count) =>
+            new StringBuilder(value.Length * count).Insert(0, value, count).ToString();
+    }
+}