[cssom] Should `insertRule()` return or throw a syntax error?

[cdoublev]

https://drafts.csswg.org/cssom-1/#dom-cssstylesheet-insertrule

3. Let parsed rule be the return value of invoking parse a rule with rule.
4. If parsed rule is a syntax error, return parsed rule.

All mainstream browsers do not return a syntax error but throws a SyntaxError DOMException when the result of parse a rule is a syntax error:

try {
  document.styleSheets[0].insertRule(' ')
} catch (error) {
  error.name // SyntaxError
  error // DOMException: Failed to execute 'insertRule' on 'CSSStyleSheet': Failed to parse the rule ' '.
}

Furthermore, the Web IDL definition does not allow to return syntax error:

  unsigned long insertRule(CSSOMString rule, optional unsigned long index = 0);

Should the current procedure be updated to this interopable behavior?

[tabatkins]

Yeah, that's just an incoherent line. The "syntax error" isn't an reifiable object in any way, just a spec concept to indicate that the parsing failed. It should indeed explicitly throw a SyntaxError DOMException.