This is a testharness.js-based test.
[FAIL] Custom properties are not allowed in the outer scope of @keyframes rules
  assert_equals: expected "@keyframes foo {\\n  0% { --valid-var: 4; }\\n  100% { left: 10px; }\\n}" but got "@keyframes foo { \\n  100% { left: 10px; }\\n}"
Harness: the test ran to completion.