chromium/third_party/blink/web_tests/fast/borders/border-image-parsing-support.html

<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<script>
test(() => {
  assert_false(CSS.supports('border-image', '1 / 1 2 3 4 5'));
  assert_false(CSS.supports('border-image', '1 / 1 / '));
  assert_true(CSS.supports('border-image', '1 / 1 / 1'));
}, 'border-image parsing support');
</script>