css/css-properties-values-api/conditional-rules.html

<!DOCTYPE html>
<link rel="help" href="https://drafts.css-houdini.org/css-properties-values-api-1/#conditional-rules">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<script>
  CSS.registerProperty({
    name: '--length',
    syntax: '<length>',
    initialValue: '0px',
    inherits: false
  });
</script>

<style>
  #target { color: red; }
  @supports(--length: green) {
    #target { color: rgb(1, 2, 3); }
  }
</style>

<div id=target></div>

<script>

test(function() {
  let cs = getComputedStyle(target);
  assert_equals(cs.getPropertyValue('color'), 'rgb(1, 2, 3)');
}, '@supports should ignore registered syntax');

test(function() {
  assert_true(CSS.supports('--length: red'));
  assert_true(CSS.supports('--length: 10px'));
  assert_true(CSS.supports('--length: anything, really'));
}, 'CSS.supports(conditionText) should ignore registered syntax');

test(function() {
  assert_true(CSS.supports('--length', 'red'));
  assert_true(CSS.supports('--length', '10px'));
  assert_true(CSS.supports('--length', ' anything, really'));
}, 'CSS.supports(property, value) should ignore registered syntax');

</script>