Why is the second parameter to MutationObserver.observe() optional?

[cdumez]

The second parameter to MutationObserver.observe() is optional in the specification. I don't understand why we don't simply mark it as mandatory considering that it will throw a TypeError if omitted anyway.

Behavior of browser engines:

• Gecko agrees with the spec (Argument is optional but throws a TypeError when omitted)
• WebKit / Blink mark the second parameter as mandatory

[annevk]

Per IDL you cannot have mandatory dictionaries as final argument. They are treated identically to {}. That is a little weird if the dictionary ends up being effectively required as is the case here, but marking it mandatory doesn't really help us since we still have to handle the {} case anyway.

See whatwg/webidl#130 for more discussion.