Intent to unship: XMLDocument.load and XMLDocument.async APIs

[Ehsan Akhgari]

Hi everyone,

As of Firefox 68 I'm planning to turn off support for non-standard
XMLDocument.load and XMLDocument.async APIs. These are really old APIs
that allow developers to load an XML document over the network
synchronously or asynchronously but were never standardized and implemented
by other web engines, and have long been replaced by XMLHttpRequest.

On Beta65, XMLDocument.load() is being used on ~0.003% of top level page
loads (see UseOfDOM3LoadMethod in
https://georgf.github.io/usecounters/index.html#kind=page&group=DEPRECATED&channel=beta&version=65)
and the setter of XMLDocument.async is used on ~0.009% (
https://georgf.github.io/usecounters/index.html#kind=page&group=XMLDOCUMENT&channel=beta&version=65).
These numbers are a bit surprising since it looks like about three times as
many pages set the async attribute without calling load() but setting async
without calling load() has no side effects...

We believe the usage is low enough to try to unship, but we're not yet
removing the code. The unshipping is done by hiding both APIs behind a
pref in https://bugzilla.mozilla.org/show_bug.cgi?id=332175 and
https://bugzilla.mozilla.org/show_bug.cgi?id=1328138.

Please let me know if you have any questions or concerns.

Thanks,
--
Ehsan