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+<!doctype html>
+<title>CSS Selectors: :is() and :where() parsing</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#matches">
+<link rel="help" href="https://drafts.csswg.org/selectors-4/#zero-matches">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style id="test-sheet">
+  random-selector { color: blue; }
+</style>
+<script>
+  let rule = document.getElementById("test-sheet").sheet.cssRules[0];
+  function assert_valid(expected_valid, pattern, expected_pattern, description) {
+    test(function() {
+      for (let pseudo of ["is", "where"]) {
+        let selector = pattern.replace("{}", ":" + pseudo);
+        let expected_selector = selector;
+        if (expected_pattern != null)
+          expected_selector = expected_pattern.replace("{}", ":" + pseudo);
+        rule.selectorText = "random-selector";
+        rule.selectorText = selector;
+        (expected_valid ? assert_equals : assert_not_equals)(
+          rule.selectorText,
+          expected_selector,
+          `${description}: ${selector}`
+        );
+      }
+    }, description);
+  }
+
+  assert_valid(true, "{}(div )", "{}(div)", "Trailing whitespace");
+  assert_valid(true, "{}(div + bar, div ~ .baz)", null, "Multiple selectors with combinators");
+
+  assert_valid(true, "{}(:is(div))", null, "Nested :is");
+  assert_valid(true, "{}(:where(div))", null, "Nested :where");
+
+  assert_valid(true, ":host({}(div))", null, "Nested inside :host, without combinators");
+  assert_valid(true, ":host({}(div ))", ":host({}(div))", "Nested inside :host, with trailing whitespace");
+  // See https://github.com/w3c/csswg-drafts/issues/5093
+  assert_valid(false, ":host({}(div .foo))", null, "Nested inside :host, with combinators");
+
+  assert_valid(true, "{}(:hover, :active)", null, "Pseudo-classes inside");
+  assert_valid(true, "{}(div):hover", null, "Pseudo-classes after");
+  assert_valid(true, "{}(div)::before", null, "Pseudo-elements after");
+  assert_valid(false, "{}(::before)", null, "Pseudo-elements inside");
+
+  assert_valid(true, "{}(div) + bar", null, "Combinators after");
+  assert_valid(true, "::part(foo):is(:hover)", null, "After part with simple pseudo-class");
+  assert_valid(false, "::part(foo):is([attr='value'])", null, "After part with invalid selector after");
+
+  assert_valid(true, ":not({}(div))", null, "Nested inside :not, without combinators");
+  assert_valid(true, ":not({}(div .foo))", null, "Nested inside :not, with combinators");
+</script>