diff --git a/layout/style/test/test_selectors.html b/layout/style/test/test_selectors.html
index 2dfba7b37a565..d688139d3c513 100644
--- a/layout/style/test/test_selectors.html
+++ b/layout/style/test/test_selectors.html
@@ -1299,14 +1299,15 @@
     test_balanced_unparseable("::-moz-color-swatch:hover#foo");
     test_balanced_unparseable(".foo::after:not(.bar) ~ h3");
 
-    for (let [selector, expected_serialization_when_parseable] of [
-      ["::-moz-color-swatch:where(.foo)", "::-moz-color-swatch:where()"],
-      ["::-moz-color-swatch:is(.foo)", "::-moz-color-swatch:is()"],
-      ["::-moz-color-swatch:where(p, :hover)", "::-moz-color-swatch:where(:hover)"],
-      ["::-moz-color-swatch:is(p, :hover)", "::-moz-color-swatch:is(:hover)"],
+    for (let selector of [
+      "::-moz-color-swatch:where(.foo)",
+      "::-moz-color-swatch:is(.foo)",
+      "::-moz-color-swatch:where(p, :hover)",
+      "::-moz-color-swatch:is(p, :hover)",
     ]) {
       test_parseable(selector);
-      should_serialize_to(selector, expected_serialization_when_parseable);
+      should_serialize_to(selector, selector);
+      ok(!CSS.supports(`selector(${selector})`), "supports should report false for forging selector parse failure");
     }
 
     run_deferred_tests();
diff --git a/servo/components/malloc_size_of/lib.rs b/servo/components/malloc_size_of/lib.rs
index f3a254fc95b3a..b2b44cad4cd8e 100644
--- a/servo/components/malloc_size_of/lib.rs
+++ b/servo/components/malloc_size_of/lib.rs
@@ -771,7 +771,8 @@ where
             Component::ParentSelector |
             Component::Nth(..) |
             Component::Host(None) |
-            Component::RelativeSelectorAnchor => 0,
+            Component::RelativeSelectorAnchor |
+            Component::Invalid(..) => 0,
         }
     }
 }
diff --git a/servo/components/selectors/builder.rs b/servo/components/selectors/builder.rs
index a24b97db16855..e386175101bf7 100644
--- a/servo/components/selectors/builder.rs
+++ b/servo/components/selectors/builder.rs
@@ -351,7 +351,8 @@ where
             Component::ExplicitNoNamespace |
             Component::DefaultNamespace(..) |
             Component::Namespace(..) |
-            Component::RelativeSelectorAnchor => {
+            Component::RelativeSelectorAnchor |
+            Component::Invalid(..) => {
                 // Does not affect specificity
             },
         }
diff --git a/servo/components/selectors/matching.rs b/servo/components/selectors/matching.rs
index be1c50f79e9bf..54b448e0da7dc 100644
--- a/servo/components/selectors/matching.rs
+++ b/servo/components/selectors/matching.rs
@@ -1100,6 +1100,7 @@ where
             );
             anchor.map_or(false, |a| a == element.opaque())
         },
+        Component::Invalid(..) => false,
     }
 }
 
diff --git a/servo/components/selectors/parser.rs b/servo/components/selectors/parser.rs
index f9d9584e6d0e1..4a83f1b25d0c2 100644
--- a/servo/components/selectors/parser.rs
+++ b/servo/components/selectors/parser.rs
@@ -18,7 +18,7 @@ use cssparser::{BasicParseError, BasicParseErrorKind, ParseError, ParseErrorKind
 use cssparser::{CowRcStr, Delimiter, SourceLocation};
 use cssparser::{Parser as CssParser, ToCss, Token};
 use precomputed_hash::PrecomputedHash;
-use servo_arc::{ThinArc, UniqueArc};
+use servo_arc::{Arc, ThinArc, UniqueArc};
 use smallvec::SmallVec;
 use std::borrow::{Borrow, Cow};
 use std::fmt::{self, Debug};
@@ -371,6 +371,7 @@ impl SelectorKey {
 }
 
 /// Whether or not we're using forgiving parsing mode
+#[derive(PartialEq)]
 enum ForgivingParsing {
     /// Discard the entire selector list upon encountering any invalid selector.
     /// This is the default behavior for almost all of CSS.
@@ -443,34 +444,28 @@ impl<Impl: SelectorImpl> SelectorList<Impl> {
         P: Parser<'i, Impl = Impl>,
     {
         let mut values = SmallVec::new();
+        let forgiving = recovery == ForgivingParsing::Yes && parser.allow_forgiving_selectors();
         loop {
-            let selector = input.parse_until_before(Delimiter::Comma, |i| {
-                parse_selector(parser, i, state, parse_relative)
+            let selector = input.parse_until_before(Delimiter::Comma, |input| {
+                let start = input.position();
+                let mut selector = parse_selector(parser, input, state, parse_relative);
+                if forgiving && (selector.is_err() || input.expect_exhausted().is_err()) {
+                    input.expect_no_error_token()?;
+                    selector = Ok(Selector::new_invalid(input.slice_from(start)));
+                }
+                selector
             });
 
-            let was_ok = selector.is_ok();
-            match selector {
-                Ok(selector) => values.push(selector),
-                Err(err) => match recovery {
-                    ForgivingParsing::No => return Err(err),
-                    ForgivingParsing::Yes => {
-                        if !parser.allow_forgiving_selectors() {
-                            return Err(err);
-                        }
-                    },
-                },
-            }
+            debug_assert!(!forgiving || selector.is_ok());
+            values.push(selector?);
 
-            loop {
-                match input.next() {
-                    Err(_) => return Ok(SelectorList(values)),
-                    Ok(&Token::Comma) => break,
-                    Ok(_) => {
-                        debug_assert!(!was_ok, "Shouldn't have got a selector if getting here");
-                    },
-                }
+            match input.next() {
+                Ok(&Token::Comma) => {},
+                Ok(_) => unreachable!(),
+                Err(_) => break,
             }
         }
+        Ok(SelectorList(values))
     }
 
     /// Replaces the parent selector in all the items of the selector list.
@@ -1045,6 +1040,7 @@ impl<Impl: SelectorImpl> Selector<Impl> {
                     Combinator(..) |
                     Host(None) |
                     Part(..) |
+                    Invalid(..) |
                     RelativeSelectorAnchor => component.clone(),
                     ParentSelector => {
                         specificity += parent_specificity;
@@ -1168,6 +1164,65 @@ impl<Impl: SelectorImpl> Selector<Impl> {
 
         true
     }
+
+    /// Parse a selector, without any pseudo-element.
+    #[inline]
+    pub fn parse<'i, 't, P>(
+        parser: &P,
+        input: &mut CssParser<'i, 't>,
+    ) -> Result<Self, ParseError<'i, P::Error>>
+    where
+        P: Parser<'i, Impl = Impl>,
+    {
+        parse_selector(
+            parser,
+            input,
+            SelectorParsingState::empty(),
+            ParseRelative::No,
+        )
+    }
+
+    pub fn new_invalid(s: &str) -> Self {
+        fn check_for_parent(input: &mut CssParser, has_parent: &mut bool) {
+            while let Ok(t) = input.next() {
+                match *t {
+                    Token::Function(_) |
+                    Token::ParenthesisBlock |
+                    Token::CurlyBracketBlock |
+                    Token::SquareBracketBlock => {
+                        let _ = input.parse_nested_block(|i| -> Result<(), ParseError<'_, BasicParseError>> {
+                            check_for_parent(i, has_parent);
+                            Ok(())
+                        });
+                    },
+                    Token::Delim('&') => {
+                        *has_parent = true;
+                    }
+                    _ => {},
+                }
+                if *has_parent {
+                    break;
+                }
+            }
+        }
+        let mut has_parent = false;
+        {
+            let mut parser = cssparser::ParserInput::new(s);
+            let mut parser = CssParser::new(&mut parser);
+            check_for_parent(&mut parser, &mut has_parent);
+        }
+        Self(ThinArc::from_header_and_iter(
+            SpecificityAndFlags {
+                specificity: 0,
+                flags: if has_parent {
+                    SelectorFlags::HAS_PARENT
+                } else {
+                    SelectorFlags::empty()
+                },
+            },
+            std::iter::once(Component::Invalid(Arc::new(String::from(s.trim()))))
+        ))
+    }
 }
 
 #[derive(Clone)]
@@ -1831,6 +1886,8 @@ pub enum Component<Impl: SelectorImpl> {
     ///
     /// Same comment as above re. the argument.
     Has(Box<[RelativeSelector<Impl>]>),
+    /// An invalid selector inside :is() / :where().
+    Invalid(Arc<String>),
     /// An implementation-dependent pseudo-element selector.
     PseudoElement(#[shmem(field_bound)] Impl::PseudoElement),
 
@@ -2369,6 +2426,7 @@ impl<Impl: SelectorImpl> ToCss for Component<Impl> {
                 dest.write_str(")")
             },
             NonTSPseudoClass(ref pseudo) => pseudo.to_css(dest),
+            Invalid(ref css) => dest.write_str(css),
             RelativeSelectorAnchor => Ok(()),
         }
     }
@@ -2519,25 +2577,6 @@ fn try_parse_combinator<'i, 't, P, Impl>(input: &mut CssParser<'i, 't>) -> Resul
     }
 }
 
-impl<Impl: SelectorImpl> Selector<Impl> {
-    /// Parse a selector, without any pseudo-element.
-    #[inline]
-    pub fn parse<'i, 't, P>(
-        parser: &P,
-        input: &mut CssParser<'i, 't>,
-    ) -> Result<Self, ParseError<'i, P::Error>>
-    where
-        P: Parser<'i, Impl = Impl>,
-    {
-        parse_selector(
-            parser,
-            input,
-            SelectorParsingState::empty(),
-            ParseRelative::No,
-        )
-    }
-}
-
 /// * `Err(())`: Invalid selector, abort
 /// * `Ok(false)`: Not a type selector, could be something else. `input` was not consumed.
 /// * `Ok(true)`: Length 0 (`*|*`), 1 (`*|E` or `ns|*`) or 2 (`|E` or `ns|E`)
@@ -4202,7 +4241,7 @@ pub mod tests {
 
         assert!(parse("foo:where()").is_ok());
         assert!(parse("foo:where(div, foo, .bar baz)").is_ok());
-        assert!(parse_expected("foo:where(::before)", Some("foo:where()")).is_ok());
+        assert!(parse("foo:where(::before)").is_ok());
     }
 
     #[test]
diff --git a/testing/web-platform/meta/css/css-nesting/cssom.html.ini b/testing/web-platform/meta/css/css-nesting/cssom.html.ini
deleted file mode 100644
index 14751f5bb48a7..0000000000000
--- a/testing/web-platform/meta/css/css-nesting/cssom.html.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[cssom.html]
-  [Simple CSSOM manipulation of subrules 10]
-    expected: FAIL
diff --git a/testing/web-platform/meta/css/css-nesting/nest-containing-forgiving.html.ini b/testing/web-platform/meta/css/css-nesting/nest-containing-forgiving.html.ini
deleted file mode 100644
index 5bc1769b4f1ca..0000000000000
--- a/testing/web-platform/meta/css/css-nesting/nest-containing-forgiving.html.ini
+++ /dev/null
@@ -1,2 +0,0 @@
-[nest-containing-forgiving.html]
-  expected: FAIL
diff --git a/testing/web-platform/meta/css/css-nesting/parsing.html.ini b/testing/web-platform/meta/css/css-nesting/parsing.html.ini
deleted file mode 100644
index b3a26082ad15e..0000000000000
--- a/testing/web-platform/meta/css/css-nesting/parsing.html.ini
+++ /dev/null
@@ -1 +0,0 @@
-[parsing.html]
diff --git a/testing/web-platform/meta/css/css-scoping/slotted-invalidation.html.ini b/testing/web-platform/meta/css/css-scoping/slotted-invalidation.html.ini
deleted file mode 100644
index e15e183630954..0000000000000
--- a/testing/web-platform/meta/css/css-scoping/slotted-invalidation.html.ini
+++ /dev/null
@@ -1,4 +0,0 @@
-[slotted-invalidation.html]
-  expected:
-    if (os == "android") and debug and fission: [OK, TIMEOUT]
-    if (os == "android") and debug and not fission: [OK, TIMEOUT]
diff --git a/testing/web-platform/meta/css/css-scoping/slotted-link.html.ini b/testing/web-platform/meta/css/css-scoping/slotted-link.html.ini
deleted file mode 100644
index 36f719091964e..0000000000000
--- a/testing/web-platform/meta/css/css-scoping/slotted-link.html.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[slotted-link.html]
-  expected:
-    if (os == "android") and fission: [OK, TIMEOUT]
diff --git a/testing/web-platform/meta/css/css-scoping/slotted-matches.html.ini b/testing/web-platform/meta/css/css-scoping/slotted-matches.html.ini
deleted file mode 100644
index a7914b8393bea..0000000000000
--- a/testing/web-platform/meta/css/css-scoping/slotted-matches.html.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[slotted-matches.html]
-  expected:
-    if (os == "android") and fission: [OK, TIMEOUT]
diff --git a/testing/web-platform/meta/css/css-scoping/slotted-nested.html.ini b/testing/web-platform/meta/css/css-scoping/slotted-nested.html.ini
deleted file mode 100644
index c63c72152b579..0000000000000
--- a/testing/web-platform/meta/css/css-scoping/slotted-nested.html.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[slotted-nested.html]
-  expected:
-    if (os == "android") and fission: [OK, TIMEOUT]
diff --git a/testing/web-platform/meta/css/css-scoping/slotted-parsing.html.ini b/testing/web-platform/meta/css/css-scoping/slotted-parsing.html.ini
deleted file mode 100644
index 60029274100ee..0000000000000
--- a/testing/web-platform/meta/css/css-scoping/slotted-parsing.html.ini
+++ /dev/null
@@ -1,15 +0,0 @@
-[slotted-parsing.html]
-  expected:
-    if (os == "android") and debug and fission: [OK, TIMEOUT]
-    if (os == "android") and debug and not fission: [OK, TIMEOUT]
-  ["::slotted(*):is(:hover)" should be a valid selector]
-    expected: FAIL
-
-  ["::slotted(*):is(#id)" should be a valid selector]
-    expected: FAIL
-
-  ["::slotted(*):where(:hover)" should be a valid selector]
-    expected: FAIL
-
-  ["::slotted(*):where(#id)" should be a valid selector]
-    expected: FAIL
diff --git a/testing/web-platform/meta/css/css-scoping/slotted-slot.html.ini b/testing/web-platform/meta/css/css-scoping/slotted-slot.html.ini
deleted file mode 100644
index c504c85f60c91..0000000000000
--- a/testing/web-platform/meta/css/css-scoping/slotted-slot.html.ini
+++ /dev/null
@@ -1,4 +0,0 @@
-[slotted-slot.html]
-  expected:
-    if (os == "android") and debug and fission: [OK, TIMEOUT]
-    if (os == "android") and debug and not fission: [OK, TIMEOUT]
diff --git a/testing/web-platform/meta/css/selectors/is-nested.html.ini b/testing/web-platform/meta/css/selectors/is-nested.html.ini
deleted file mode 100644
index 333bbb79dbdc7..0000000000000
--- a/testing/web-platform/meta/css/selectors/is-nested.html.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[is-nested.html]
-  expected:
-    if (os == "android") and fission: [OK, TIMEOUT]
diff --git a/testing/web-platform/meta/css/selectors/is-where-basic.html.ini b/testing/web-platform/meta/css/selectors/is-where-basic.html.ini
deleted file mode 100644
index 09dd763bb8ea3..0000000000000
--- a/testing/web-platform/meta/css/selectors/is-where-basic.html.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[is-where-basic.html]
-  expected:
-    if (os == "android") and fission: [OK, TIMEOUT]
diff --git a/testing/web-platform/meta/css/selectors/is-where-error-recovery.html.ini b/testing/web-platform/meta/css/selectors/is-where-error-recovery.html.ini
deleted file mode 100644
index d731f88945926..0000000000000
--- a/testing/web-platform/meta/css/selectors/is-where-error-recovery.html.ini
+++ /dev/null
@@ -1,5 +0,0 @@
-[is-where-error-recovery.html]
-  expected:
-    if (os == "android") and fission: [OK, TIMEOUT]
-  [CSS Selectors: :is() and :where() error recovery]
-    expected: FAIL
diff --git a/testing/web-platform/meta/css/selectors/is-where-not.html.ini b/testing/web-platform/meta/css/selectors/is-where-not.html.ini
deleted file mode 100644
index 071bf7acd2f1c..0000000000000
--- a/testing/web-platform/meta/css/selectors/is-where-not.html.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[is-where-not.html]
-  expected:
-    if (os == "android") and fission: [OK, TIMEOUT]
diff --git a/testing/web-platform/meta/css/selectors/is-where-parsing.html.ini b/testing/web-platform/meta/css/selectors/is-where-parsing.html.ini
deleted file mode 100644
index e7320dbe60f06..0000000000000
--- a/testing/web-platform/meta/css/selectors/is-where-parsing.html.ini
+++ /dev/null
@@ -1,5 +0,0 @@
-[is-where-parsing.html]
-  expected:
-    if (os == "android") and fission: [OK, TIMEOUT]
-  [Pseudo-elements inside]
-    expected: FAIL
diff --git a/testing/web-platform/meta/css/selectors/is-where-pseudo-classes.html.ini b/testing/web-platform/meta/css/selectors/is-where-pseudo-classes.html.ini
deleted file mode 100644
index deffa76608c95..0000000000000
--- a/testing/web-platform/meta/css/selectors/is-where-pseudo-classes.html.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[is-where-pseudo-classes.html]
-  expected:
-    if (os == "android") and fission: [OK, TIMEOUT]
diff --git a/testing/web-platform/meta/css/selectors/is-where-shadow.html.ini b/testing/web-platform/meta/css/selectors/is-where-shadow.html.ini
index cbcea64fe1cca..99885484ab114 100644
--- a/testing/web-platform/meta/css/selectors/is-where-shadow.html.ini
+++ b/testing/web-platform/meta/css/selectors/is-where-shadow.html.ini
@@ -1,5 +1,3 @@
 [is-where-shadow.html]
-  expected:
-    if (os == "android") and fission: [OK, TIMEOUT]
   [:is() inside :host-context()]
     expected: FAIL
diff --git a/testing/web-platform/tests/css/css-scoping/slotted-parsing.html b/testing/web-platform/tests/css/css-scoping/slotted-parsing.html
index bed4dedd56072..e4657b588af5e 100644
--- a/testing/web-platform/tests/css/css-scoping/slotted-parsing.html
+++ b/testing/web-platform/tests/css/css-scoping/slotted-parsing.html
@@ -27,13 +27,14 @@
   test_valid_selector("::slotted([attr]:hover)");
   test_valid_selector("::slotted(:not(.a))");
 
-  test_valid_selector("::slotted(*):is()");
-  test_valid_selector("::slotted(*):is(:hover)");
-  test_valid_selector("::slotted(*):is(#id)");
+  test_valid_forgiving_selector("::slotted(*):is()");
+  test_valid_forgiving_selector("::slotted(*):is(:hover)");
+  test_valid_forgiving_selector("::slotted(*):is(#id)");
 
-  test_valid_selector("::slotted(*):where()");
-  test_valid_selector("::slotted(*):where(:hover)");
-  test_valid_selector("::slotted(*):where(#id)");
+  test_valid_forgiving_selector("::slotted(*):where()");
+  test_valid_forgiving_selector("::slotted(*):where(:hover)");
+  test_valid_forgiving_selector("::slotted(*):where(#id)");
+  test_valid_forgiving_selector("::slotted(*):where(::before)");
 
   // Allow tree-abiding pseudo elements after ::slotted
   test_valid_selector("::slotted(*)::before");
diff --git a/testing/web-platform/tests/css/selectors/is-where-parsing.html b/testing/web-platform/tests/css/selectors/is-where-parsing.html
index 3159ecfe6a65b..6d404dd953b28 100644
--- a/testing/web-platform/tests/css/selectors/is-where-parsing.html
+++ b/testing/web-platform/tests/css/selectors/is-where-parsing.html
@@ -5,27 +5,20 @@
 <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 src="/css/support/parsing-testcommon.js"></script>
 <script>
-  let rule = document.getElementById("test-sheet").sheet.cssRules[0];
-  function assert_valid(expected_parseable, 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_parseable ? assert_equals : assert_not_equals)(
-          rule.selectorText,
-          expected_selector,
-          `${description}: ${selector}`
-        );
+  function assert_valid(valid, pattern, expected_pattern, description) {
+    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);
+      if (valid) {
+        test_valid_selector(selector, expected_selector);
+      } else {
+        test_valid_forgiving_selector(selector);
       }
-    }, description);
+    }
   }
 
   assert_valid(true, "{}(div )", "{}(div)", "Trailing whitespace");
@@ -42,12 +35,11 @@
   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");
-  // Should ask clarification from CSSWG
-  assert_valid(true, "{}(::before)", null, "Pseudo-elements inside");
+  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, "::part(foo){}(:hover)", null, "After part with simple pseudo-class");
+  assert_valid(false, "::part(foo){}([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");
diff --git a/testing/web-platform/tests/css/selectors/parsing/parse-has-disallow-nesting-has-inside-has.html b/testing/web-platform/tests/css/selectors/parsing/parse-has-disallow-nesting-has-inside-has.html
index a633c913d1601..8ea816b5d898e 100644
--- a/testing/web-platform/tests/css/selectors/parsing/parse-has-disallow-nesting-has-inside-has.html
+++ b/testing/web-platform/tests/css/selectors/parsing/parse-has-disallow-nesting-has-inside-has.html
@@ -8,6 +8,8 @@
 <script src="/css/support/parsing-testcommon.js"></script>
 <script>
   test_invalid_selector('.a:has(.b:has(.c))');
+  test_valid_forgiving_selector(':has(:is(:has(*)))');
+  test_valid_forgiving_selector(':has(:where(:has(*)))');
   test(() => {
     // It's not easy to check that these are invalid because :is() and :where()
     // use forgiving parsing. Check that they never match instead.
diff --git a/testing/web-platform/tests/css/support/parsing-testcommon.js b/testing/web-platform/tests/css/support/parsing-testcommon.js
index e480f26b3f3db..93fd0090770b5 100644
--- a/testing/web-platform/tests/css/support/parsing-testcommon.js
+++ b/testing/web-platform/tests/css/support/parsing-testcommon.js
@@ -50,10 +50,14 @@ function test_invalid_value(property, value) {
     }, "e.style['" + property + "'] = " + stringifiedValue + " should not set the property value");
 }
 
+function test_valid_forgiving_selector(selector) {
+  test_valid_selector(selector, selector, { onlyWhenForgiving: true });
+}
+
 // serializedSelector can be the expected serialization of selector,
 // or an array of permitted serializations,
 // or omitted if value should serialize as selector.
-function test_valid_selector(selector, serializedSelector) {
+function test_valid_selector(selector, serializedSelector, options) {
     if (arguments.length < 2)
         serializedSelector = selector;
 
@@ -85,6 +89,9 @@ function test_valid_selector(selector, serializedSelector) {
         assert_equals(cssRules.length, 1, "Sheet should have 1 rule");
 
         assert_equals(cssRules[0].selectorText, readSelector, "serialization should round-trip");
+
+        let supports = !options?.onlyWhenForgiving;
+        assert_equals(CSS.supports(`selector(${selector})`), supports, "CSS.supports() reports the expected value");
     }, stringifiedSelector + " should be a valid selector");
 }