From 71c4fce87fb30c6e6e55fedb2d458e643cb27b2d Mon Sep 17 00:00:00 2001
From: Karl Seguin <k@openmymind.io>
Date: Mon, 9 Mar 2026 16:39:44 +0800
Subject: [PATCH] Empty :is() and :where() pseudoselectors are valid (and
 return nothing)

---
 src/browser/tests/element/pseudo_classes.html   | 11 +++++++++++
 src/browser/tests/element/selector_invalid.html |  2 --
 src/browser/webapi/selector/Parser.zig          |  4 ++--
 3 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/src/browser/tests/element/pseudo_classes.html b/src/browser/tests/element/pseudo_classes.html
index fe75ab84..355d2ee7 100644
--- a/src/browser/tests/element/pseudo_classes.html
+++ b/src/browser/tests/element/pseudo_classes.html
@@ -81,6 +81,17 @@
 }
 </script>
 
+<script id="is_empty">
+{
+    // Empty :is() and :where() are valid per spec and match nothing
+    const isEmptyResult = document.querySelectorAll(':is()');
+    testing.expectEqual(0, isEmptyResult.length);
+
+    const whereEmptyResult = document.querySelectorAll(':where()');
+    testing.expectEqual(0, whereEmptyResult.length);
+}
+</script>
+
 <div id=escaped class=":popover-open"></div>
 <script id="escaped">
 {
diff --git a/src/browser/tests/element/selector_invalid.html b/src/browser/tests/element/selector_invalid.html
index 38c68671..35409c19 100644
--- a/src/browser/tests/element/selector_invalid.html
+++ b/src/browser/tests/element/selector_invalid.html
@@ -12,8 +12,6 @@
     // Empty functional pseudo-classes should error
     testing.expectError("Error: InvalidPseudoClass", () => container.querySelector(':has()'));
     testing.expectError("Error: InvalidPseudoClass", () => container.querySelector(':not()'));
-    testing.expectError("Error: InvalidPseudoClass", () => container.querySelector(':is()'));
-    testing.expectError("Error: InvalidPseudoClass", () => container.querySelector(':where()'));
     testing.expectError("Error: InvalidPseudoClass", () => container.querySelector(':lang()'));
 }
 </script>
diff --git a/src/browser/webapi/selector/Parser.zig b/src/browser/webapi/selector/Parser.zig
index 376bbcdc..f9b1bb23 100644
--- a/src/browser/webapi/selector/Parser.zig
+++ b/src/browser/webapi/selector/Parser.zig
@@ -487,7 +487,7 @@ fn pseudoClass(self: *Parser, arena: Allocator, page: *Page) !Selector.PseudoCla
             if (self.peek() != ')') return error.InvalidPseudoClass;
             self.input = self.input[1..];
 
-            if (selectors.items.len == 0) return error.InvalidPseudoClass;
+            // Empty :is() is valid per spec - matches nothing
             return .{ .is = selectors.items };
         }
 
@@ -514,7 +514,7 @@ fn pseudoClass(self: *Parser, arena: Allocator, page: *Page) !Selector.PseudoCla
             if (self.peek() != ')') return error.InvalidPseudoClass;
             self.input = self.input[1..];
 
-            if (selectors.items.len == 0) return error.InvalidPseudoClass;
+            // Empty :where() is valid per spec - matches nothing
             return .{ .where = selectors.items };
         }