Align CSSStyleDeclaration.setProperty() with the specification: - https://drafts.csswg.org/cssom/#the-cssstyledeclaration-interface In particular: 1. The 'value' parameter should not be optional 2. The 'priority' parameter should treat null as the empty string rather than the string "null" 3. The 'priority' parameter's default value should be the empty string, not the string "undefined" 4. CSSStyleDeclaration.setProperty() should return early if 'priority' is not the empty string and is not an ASCII case-insensitive match for the string "important". Chrome matches the specification entirely. Firefox matches the specification with the exception that it does a case-sensitive match for "important".