Value.parse/parseAll results in a CSSStyleValue with
// the 'expected' type.
function verify_parsed_type(prop, value, expected) {
    let parse_value = CSSStyleValue.parse(prop, value);
    let parse_all_value = CSSStyleValue.parseAll(prop, value);

    assert_true(parse_value instanceof expected);
    assert_true(parse_all_value.every(x => x instanceof expected))
}

// On the target element, verify that computed value of 'name' is an instance
// of 'expected' and not an instance of CSSUnparsedValue.
//
// 