Re: Should MutationObservers be able to observe work done by the HTML parser?

On Fri, Jun 29, 2012 at 2:44 AM, Jonas Sicking <jonas@sicking.cc> wrote:

> All in all I think that as soon as we introduce exceptions to when
> MutationObserver callbacks fire we change the API from being a
> reliable way to track DOM mutations to a unreliable way where all
> callers have to be aware of exceptions and deal with them in other
> ways. I.e. it feels like it significantly reduces the value of
> MutationObservers.
>
> And so far I don't see any good arguments made for making that
> reduction in value. Did I miss any arguments other then the randomness
> argument?
>

Performance was one concern that's come up in discussing this with Ojan and
Rafael. Imagine a MutationObserver attached to the document near the top of
the page. Now we need to create basically one MutationRecord per node
inserted (because the parser operates in a depth-first sort of order). I'm
not at all sure this is a show-stopper (we might be able to introduce some
new MutationRecord type that could compactly represent parser-style
operation), but it definitely seems worrisome, especially given that one of
the common uses for MutationObservers is extensions which might run on many
(all?) pages.

- Adam