Consider changing the spec text to allow buffered with entryTypes

[hiikezoe]

The spec text in question is here in observe() method section at the 3

3. If options's entryTypes is present and any other member is also present, then throw a "TypeError".

The text means the correct behavior here is to throw a TypeError if there are both of buffered and entryTypes, but either Blink or WebKit doesn't throw the exception. It behaves as if there's no buffered option specified.

On the other hand, Gecko does throw the exception. But it is going to be changed to align with the other browser engine since there's a severe bug report (bug 1915589) that a shopping site isn't usable at all because of the exception.

That's being said, I personally prefer the throwing the exception behavior because from web developers perspective buffered can be specified with entryTypes but buffered option is ignored, it would be confusing. So I am okay to keep this issue open until Chrome's usage counter of the case reaches to a point that we can throw the exception in the case.

CCing @noamr

There's an open interop 2025 for this behavior difference. web-platform-tests/interop#856