Register the ::marker pseudo-element

src/ExCSS.Tests/SelectorsTests.cs

@@ -146,6 +146,31 @@ public void NthChildRejectsMalformedSpacedOffset(string selector)
         Assert.Equal(0, sheet.Rules.Length);
     }
 
+    [Theory]
+    // ::marker is a standard pseudo-element (CSS Lists 3 6.1). It was unregistered, so a rule using it
+    // was rejected wholesale rather than parsed. Only the two-colon form exists - unlike ::before/::after,
+    // ::marker has no one-colon legacy spelling.
+    [InlineData("li::marker", "li::marker")]
+    [InlineData("::marker", "::marker")]
+    public void MarkerPseudoElementIsParsed(string selector, string expectedText)
+    {
+        var sheet = new StylesheetParser().Parse(selector + " { color: red }");
+
+        Assert.Equal(1, sheet.Rules.Length);
+        Assert.Equal(expectedText, ((StyleRule)sheet.Rules[0]).SelectorText);
+    }
+
+    [Fact]
+    public void MarkerPseudoElementProducesPseudoElementSelector()
+    {
+        var sheet = new StylesheetParser().Parse("li::marker { color: red }");
+        var selector = ((StyleRule)sheet.Rules[0]).Selector;
+        var subject = selector is CompoundSelector compound ? compound.Last() : selector;
+
+        var pseudo = Assert.IsType<PseudoElementSelector>(subject);
+        Assert.Equal("marker", pseudo.Name);
+    }
+
     private static bool HasStandardPseudoElementSelector(ISelector selector, bool negate = false)
     {
         if (selector is PseudoElementSelector pes)

src/ExCSS/Enumerations/PseudoElementNames.cs

@@ -4,6 +4,7 @@ public static class PseudoElementNames
     {
         public static readonly string Before = "before";
         public static readonly string After = "after";
+        public static readonly string Marker = "marker";
         public static readonly string Selection = "selection";
         public static readonly string FirstLine = "first-line";
         public static readonly string FirstLetter = "first-letter";

src/ExCSS/Factories/PseudoElementSelectorFactory.cs

@@ -20,6 +20,7 @@ public sealed class PseudoElementSelectorFactory
                     // some implementations are dubious (first-line, first-letter, ...)
                     PseudoElementNames.Before,
                     PseudoElementNames.After,
+                    PseudoElementNames.Marker,
                     PseudoElementNames.Selection,
                     PseudoElementNames.FirstLine,
                     PseudoElementNames.FirstLetter,