diff --git a/src/ExCSS.Tests/CssNestingTests.cs b/src/ExCSS.Tests/CssNestingTests.cs
new file mode 100644
index 0000000..f5f9a44
--- /dev/null
+++ b/src/ExCSS.Tests/CssNestingTests.cs
@@ -0,0 +1,130 @@
+using System.Linq;
+using Xunit;
+
+namespace ExCSS.Tests
+{
+    /// <summary>
+    /// CSS Nesting (<see href="https://www.w3.org/TR/css-nesting-1/">CSS Nesting 1</see>): the <c>&amp;</c>
+    /// nesting selector and nested style rules inside a declaration block. Nested rules are resolved at
+    /// parse time into ordinary rules with absolute selectors (<c>&amp;</c> → <c>:is(parent)</c>) and
+    /// exposed via <see cref="IStyleRule.NestedRules"/>. Includes the regression guards that the
+    /// declaration path is unaffected (hex colors, custom properties, a declaration after a nested rule).
+    /// </summary>
+    public class CssNestingTests : CssConstructionFunctions
+    {
+        private static StyleRule ParseRule(string source) =>
+            (StyleRule)ParseStyleSheet(source).Rules.First();
+
+        [Fact]
+        public void ImplicitAmpersandCompoundResolvesToIsParent()
+        {
+            // `&.active` == `:is(.box).active` — the same element carrying both classes.
+            var rule = ParseRule(".box { &.active { color: #0000ff; } }");
+            var nested = Assert.Single(rule.NestedRules);
+            Assert.Equal(":is(.box).active", nested.SelectorText);
+        }
+
+        [Fact]
+        public void ImplicitDescendantResolvesToIsParentDescendant()
+        {
+            // `& span` == `:is(.box) span` (descendant).
+            var rule = ParseRule(".box { color: #ff0000; & span { color: #0000ff; } }");
+            Assert.Equal(".box", rule.SelectorText);
+            var nested = Assert.Single(rule.NestedRules);
+            Assert.Equal(":is(.box) span", nested.SelectorText);
+        }
+
+        [Fact]
+        public void TypeSelectorNestedRuleIsNotMistakenForADeclaration()
+        {
+            // `p { ... }` inside a block starts with an Ident (like a property name) — the classifier must
+            // still see the `{` before any `;` and treat it as a nested rule.
+            var rule = ParseRule(".card { p { color: #0000ff; } }");
+            var nested = Assert.Single(rule.NestedRules);
+            Assert.Equal(":is(.card) p", nested.SelectorText);
+        }
+
+        [Fact]
+        public void NoAmpersandPreludeIsScopedUnderParent()
+        {
+            // A prelude with no `&` is made relative to the parent: `.inner` → `:is(.card) .inner`.
+            var rule = ParseRule(".card { .inner { color: #0000ff; } }");
+            var nested = Assert.Single(rule.NestedRules);
+            Assert.Equal(":is(.card) .inner", nested.SelectorText);
+        }
+
+        [Fact]
+        public void LeadingChildCombinatorIsScopedUnderParent()
+        {
+            // `> p` == `:is(.card) > p` — only a direct child p matches.
+            var rule = ParseRule(".card { > p { color: #0000ff; } }");
+            var nested = Assert.Single(rule.NestedRules);
+            Assert.Equal(":is(.card)>p", nested.SelectorText);
+        }
+
+        [Fact]
+        public void HexColorInNestedValueSurvivesTheLookahead()
+        {
+            // Regression: the classify-then-rewind must not corrupt a `#rrggbb` value (value-mode `#`
+            // tokenization) the way a token buffer would. Compare against a non-nested equivalent.
+            var nested = Assert.Single(ParseRule(".card { p { color: #123456; } }").NestedRules);
+            var direct = ParseRule("p { color: #123456; }");
+            Assert.Equal(direct.Style["color"], nested.Style["color"]);
+            Assert.NotEqual(string.Empty, nested.Style["color"]);
+        }
+
+        [Fact]
+        public void HexColorInTopLevelValueUnaffected()
+        {
+            // The whole feature must leave an ordinary (non-nested) declaration block unaffected.
+            var direct = ParseRule("p { color: #123456; }");
+            var control = ParseRule("q { color: #123456; }");
+            Assert.Equal(control.Style["color"], direct.Style["color"]);
+            Assert.Empty(direct.NestedRules);
+        }
+
+        [Fact]
+        public void DeclarationAfterNestedRuleStillApplies()
+        {
+            // A declaration written AFTER a nested rule still applies to the parent (the outer loop resumes
+            // correctly); here the later blue wins over the earlier red.
+            var rule = ParseRule(".card { color: #ff0000; & span { color: #00ff00; } color: #0000ff; }");
+            var blue = ParseRule("x { color: #0000ff; }").Style["color"];
+            Assert.Equal(blue, rule.Style["color"]);
+            Assert.Single(rule.NestedRules);
+        }
+
+        [Fact]
+        public void CustomPropertyIsNotMistakenForANestedRule()
+        {
+            // `--x: …` is always a declaration, never a nested rule, even though the classifier scans for `{`.
+            // (Custom-property value storage is a separate feature; here we only assert the `--x` declaration
+            // is not swallowed as a nested rule and the real nested rule is still captured.)
+            var rule = ParseRule(".card { --x: 1; & span { color: #0000ff; } }");
+            var nested = Assert.Single(rule.NestedRules);
+            Assert.Equal(":is(.card) span", nested.SelectorText);
+        }
+
+        [Fact]
+        public void MultiLevelNestingResolvesAgainstEachParent()
+        {
+            // `.a { & .b { & .c { … } } }` → `:is(.a) .b`, then `:is(:is(.a) .b) .c`.
+            var a = ParseRule(".a { & .b { & .c { color: #0000ff; } } }");
+            var b = Assert.Single(a.NestedRules);
+            Assert.Equal(":is(.a) .b", b.SelectorText);
+            var c = Assert.Single(b.NestedRules);
+            Assert.Equal(":is(:is(.a) .b) .c", c.SelectorText);
+        }
+
+        [Fact]
+        public void AmpersandInsideAttributeStringIsPreserved()
+        {
+            // Regression: a `&` inside an attribute-value string must NOT be substituted, and a prelude whose
+            // only `&` is inside a string is still scoped under the parent (not the replace branch).
+            var rule = ParseRule(".card { [data-x=\"a&b\"] { color: #0000ff; } }");
+            var nested = Assert.Single(rule.NestedRules);
+            Assert.StartsWith(":is(.card)", nested.SelectorText);
+            Assert.Contains("a&b", nested.SelectorText);
+        }
+    }
+}
diff --git a/src/ExCSS/Parser/LexerBase.cs b/src/ExCSS/Parser/LexerBase.cs
index b734aa6..e259528 100644
--- a/src/ExCSS/Parser/LexerBase.cs
+++ b/src/ExCSS/Parser/LexerBase.cs
@@ -42,6 +42,22 @@ public TextPosition GetCurrentPosition()
             return new(Line, Column, Position);
         }
 
+        /// <summary>
+        /// Repositions the lexer so the next token is re-lexed from character index
+        /// <paramref name="sourceIndex"/> (a raw <see cref="Source"/> index, as returned by
+        /// <see cref="InsertionPoint"/>). Unlike setting <see cref="InsertionPoint"/> (whose
+        /// <c>BackNative</c> loop is not a faithful inverse across <c>\r\n</c> normalization), this
+        /// restores the character stream exactly — line/column tracking is not rewound (it only affects
+        /// reported positions, never token content). Used by <see cref="StylesheetComposer"/> to rewind a
+        /// CSS-Nesting classification look-ahead so a declaration re-lexes cleanly in value mode.
+        /// </summary>
+        public void RewindTo(int sourceIndex)
+        {
+            Source.Index = sourceIndex;
+            // Non-EOF so the next Advance() actually reads Source[sourceIndex] rather than short-circuiting.
+            Current = Symbols.Null;
+        }
+
         protected char SkipSpaces()
         {
             var c = GetNext();
diff --git a/src/ExCSS/Parser/StylesheetComposer.cs b/src/ExCSS/Parser/StylesheetComposer.cs
index 5ab8cce..93b50e5 100644
--- a/src/ExCSS/Parser/StylesheetComposer.cs
+++ b/src/ExCSS/Parser/StylesheetComposer.cs
@@ -10,6 +10,12 @@ internal sealed class StylesheetComposer
         private readonly StylesheetParser _parser;
         private readonly Stack<StylesheetNode> _nodes;
 
+        // The source index (raw, into _lexer.Source) immediately before the most recently read token.
+        // Captured by NextToken so a CSS-Nesting classification look-ahead can rewind to a construct's
+        // exact start without any position arithmetic (which is unreliable across \r\n normalization and
+        // unicode escapes) and can slice the nested prelude's source text.
+        private int _markBeforeLastToken;
+
         public StylesheetComposer(Lexer lexer, StylesheetParser parser)
         {
             _lexer = lexer;
@@ -663,9 +669,13 @@ public TextPosition FillDeclarations(StyleDeclaration style)
                     else
                     {
                         // Advance to the next token or this is an endless loop
-                        token = _lexer.Get();
+                        token = NextToken();
                     }
                 }
+                else if (TryCreateNestedRule(ref token))
+                {
+                    // CSS Nesting: a nested style rule was parsed and attached to the enclosing rule.
+                }
                 else
                 {
                     var sourceProperty = CreateDeclarationWith(PropertyFactory.Instance.Create, ref token);
@@ -719,6 +729,168 @@ public TextPosition FillDeclarations(StyleDeclaration style)
             return token.Position;
         }
 
+        /// <summary>
+        /// CSS Nesting ([CSS Nesting 1](https://www.w3.org/TR/css-nesting-1/)): if the current construct
+        /// in a declaration block is a nested style rule (a selector prelude followed by a <c>{ }</c>
+        /// block) rather than a declaration, parses it, resolves its selector against the enclosing rule,
+        /// attaches it, and returns true (with <paramref name="token"/> advanced past the block). Returns
+        /// false for an ordinary declaration, having rewound the lexer so the caller re-reads it.
+        /// </summary>
+        private bool TryCreateNestedRule(ref Token token)
+        {
+            // Raw source index just before the current (first) token — the construct start, captured by
+            // NextToken. Faithful across \r\n normalization, unlike deriving it from token.Position.
+            var preludeStart = _markBeforeLastToken;
+
+            if (!IsNestedRuleAhead(ref token, out var braceStart))
+                return false;
+
+            CreateNestedStyleRule(preludeStart, braceStart);
+            token = NextToken();
+            return true;
+        }
+
+        /// <summary>
+        /// Looks ahead (bracket-aware) to classify the construct starting at <paramref name="token"/> as a
+        /// nested style rule vs a declaration: the first top-level <c>{</c> means a nested rule (the stream
+        /// is left just after it and its source index returned via <paramref name="braceStart"/>); a
+        /// top-level <c>;</c>/<c>}</c>/EOF means a declaration, in which case the lexer is rewound to the
+        /// construct start and <paramref name="token"/> re-read so the unchanged declaration path re-lexes
+        /// it in value mode (avoiding the <c>#</c> value-mode tokenization hazard a token buffer would hit).
+        /// Custom properties (<c>--x</c>) are always declarations, even with a <c>{</c> in their value.
+        /// Function tokens (e.g. <c>url(…)</c>) are opaque — the lexer already consumed their parentheses —
+        /// so only bare round/square brackets contribute to depth.
+        /// </summary>
+        private bool IsNestedRuleAhead(ref Token token, out int braceStart)
+        {
+            braceStart = 0;
+
+            if (token.Type == TokenType.Ident && token.Data is { } name &&
+                name.StartsWith("--", StringComparison.Ordinal))
+                return false;
+
+            var rewindMark = _markBeforeLastToken;   // raw source index before `token` (the first token)
+            var depth = 0;
+            var scan = token;
+            var scanStart = rewindMark;              // raw source index before `scan`
+
+            while (scan.Type != TokenType.EndOfFile)
+            {
+                switch (scan.Type)
+                {
+                    case TokenType.RoundBracketOpen:
+                    case TokenType.SquareBracketOpen:
+                        depth++;
+                        break;
+                    case TokenType.RoundBracketClose:
+                    case TokenType.SquareBracketClose:
+                        if (depth > 0) depth--;
+                        break;
+                    case TokenType.CurlyBracketOpen when depth == 0:
+                        braceStart = scanStart;
+                        token = scan;
+                        return true;
+                    case TokenType.CurlyBracketClose when depth == 0:
+                    case TokenType.Semicolon when depth == 0:
+                        _lexer.RewindTo(rewindMark);
+                        token = NextToken();
+                        return false;
+                }
+
+                scanStart = _lexer.InsertionPoint;   // raw index just before the next token
+                scan = _lexer.Get();
+            }
+
+            _lexer.RewindTo(rewindMark);
+            token = NextToken();
+            return false;
+        }
+
+        /// <summary>
+        /// Builds a <see cref="StyleRule"/> from a nested prelude (source span
+        /// <c>[preludeStart, braceStart)</c>) resolved against the enclosing rule's selector, consumes
+        /// its declaration block from the live stream, and attaches it to the parent rule via
+        /// <see cref="StyleRule.AddNestedRule"/>. The resolved selector is absolute (<c>&amp;</c> →
+        /// <c>:is(parent)</c>) so the cascade/matcher need no nesting awareness.
+        /// </summary>
+        private void CreateNestedStyleRule(int preludeStart, int braceStart)
+        {
+            var parent = _nodes.OfType<StyleRule>().FirstOrDefault();
+            var preludeText = _lexer.Source.Text.Substring(preludeStart, braceStart - preludeStart);
+
+            var selector = parent == null
+                ? null
+                : _parser.ParseSelector(ResolveNestedSelector(preludeText, parent.SelectorText));
+
+            // Always consume the block (via a rule pushed on _nodes so nested-within-nested resolves) to
+            // keep the parser in sync; only attach it when the selector actually resolved.
+            var rule = new StyleRule(_parser);
+
+            if (selector != null)
+                rule.Selector = selector;
+
+            _nodes.Push(rule);
+            FillDeclarations(rule.Style);
+            _nodes.Pop();
+
+            if (parent != null && selector != null)
+                parent.AddNestedRule(rule);
+        }
+
+        /// <summary>
+        /// Resolves a nested selector's prelude text against its parent's selector text per CSS Nesting:
+        /// each <c>&amp;</c> becomes <c>:is(parent)</c> (giving <c>&amp;</c> the parent's specificity); a
+        /// prelude with no <c>&amp;</c> is made relative to the parent (<c>:is(parent) &lt;prelude&gt;</c>),
+        /// which correctly yields both the implicit-descendant (<c>.b</c> → <c>:is(parent) .b</c>) and the
+        /// leading-combinator (<c>&gt; .b</c> → <c>:is(parent) &gt; .b</c>) forms.
+        /// </summary>
+        private static string ResolveNestedSelector(string prelude, string parentText)
+        {
+            var trimmed = prelude.Trim();
+            var parentIs = ":is(" + (string.IsNullOrEmpty(parentText) ? "*" : parentText) + ")";
+
+            // Substitute `&` token-aware: only a `&` that is a real nesting selector is replaced, never a
+            // `&` inside a string/attribute value (e.g. `[data-x="a&b"]`), which must be preserved. This
+            // also decides the "has &" branch correctly, so a prelude whose only `&` is inside a string is
+            // still scoped under the parent (:is(parent) <prelude>) rather than mis-taking the replace path.
+            var sb = Pool.NewStringBuilder();
+            var hasNestingSelector = false;
+            var quote = '\0';
+
+            for (var i = 0; i < trimmed.Length; i++)
+            {
+                var c = trimmed[i];
+
+                if (quote != '\0')
+                {
+                    sb.Append(c);
+                    if (c == '\\' && i + 1 < trimmed.Length)
+                        sb.Append(trimmed[++i]);   // escaped char inside a string — copy verbatim
+                    else if (c == quote)
+                        quote = '\0';
+                    continue;
+                }
+
+                switch (c)
+                {
+                    case '"':
+                    case '\'':
+                        quote = c;
+                        sb.Append(c);
+                        break;
+                    case '&':
+                        hasNestingSelector = true;
+                        sb.Append(parentIs);
+                        break;
+                    default:
+                        sb.Append(c);
+                        break;
+                }
+            }
+
+            return hasNestingSelector ? sb.ToPool() : parentIs + " " + sb.ToPool();
+        }
+
         public Property CreateDeclarationWith(Func<string, Property> createProperty, ref Token token)
         {
             var property = default(Property);
@@ -916,6 +1088,7 @@ private void JumpToDeclEnd(ref Token current)
 
         private Token NextToken()
         {
+            _markBeforeLastToken = _lexer.InsertionPoint;
             return _lexer.Get();
         }
 
@@ -942,7 +1115,7 @@ private void ParseComments(ref Token token)
                     current.AppendChild(comment);
                 }
 
-                token = _lexer.Get();
+                token = NextToken();
             }
         }
 
diff --git a/src/ExCSS/Rules/IStyleRule.cs b/src/ExCSS/Rules/IStyleRule.cs
index 95d4c30..1d2d890 100644
--- a/src/ExCSS/Rules/IStyleRule.cs
+++ b/src/ExCSS/Rules/IStyleRule.cs
@@ -1,9 +1,17 @@
-namespace ExCSS
+using System.Collections.Generic;
+
+namespace ExCSS
 {
     public interface IStyleRule : IRule
     {
         string SelectorText { get; set; }
         StyleDeclaration Style { get; }
         ISelector Selector { get; set; }
+
+        /// <summary>
+        /// CSS Nesting: style rules nested inside this rule's block, each already resolved to an
+        /// absolute selector against this (parent) rule. Empty for a non-nesting rule.
+        /// </summary>
+        IReadOnlyList<IStyleRule> NestedRules { get; }
     }
-}
\ No newline at end of file
+}
diff --git a/src/ExCSS/Rules/MarginStyleRule.cs b/src/ExCSS/Rules/MarginStyleRule.cs
index 2b06e0a..f00f9b9 100644
--- a/src/ExCSS/Rules/MarginStyleRule.cs
+++ b/src/ExCSS/Rules/MarginStyleRule.cs
@@ -1,10 +1,15 @@
-using System.IO;
+using System;
+using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 
 namespace ExCSS
 {
     public sealed class MarginStyleRule : Rule, IStyleRule
     {
+        // @page margin boxes don't participate in CSS Nesting.
+        public IReadOnlyList<IStyleRule> NestedRules => Array.Empty<IStyleRule>();
+
         public MarginStyleRule(StylesheetParser parser) : base(RuleType.Style, parser)
         {
             AppendChild(new StyleDeclaration(this));
diff --git a/src/ExCSS/Rules/StyleRule.cs b/src/ExCSS/Rules/StyleRule.cs
index ffa027d..c99df59 100644
--- a/src/ExCSS/Rules/StyleRule.cs
+++ b/src/ExCSS/Rules/StyleRule.cs
@@ -1,16 +1,26 @@
-using System.IO;
+using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 
 namespace ExCSS
 {
     public sealed class StyleRule : Rule, IStyleRule
     {
+        // CSS Nesting: rules written inside this rule's block, resolved to absolute selectors against
+        // this parent. Kept in a dedicated list (not Children) so Selector/Style lookups and ToCss are
+        // unaffected. Populated by StylesheetComposer.FillDeclarations.
+        private readonly List<IStyleRule> _nestedRules = new List<IStyleRule>();
+
         public StyleRule(StylesheetParser parser) : base(RuleType.Style, parser)
         {
             AppendChild(AllSelector.Create());
             AppendChild(new StyleDeclaration(this));
         }
 
+        public IReadOnlyList<IStyleRule> NestedRules => _nestedRules;
+
+        public void AddNestedRule(IStyleRule rule) => _nestedRules.Add(rule);
+
         public override void ToCss(TextWriter writer, IStyleFormatter formatter)
         {
             writer.Write(formatter.Style(SelectorText, Style));