Intent to unship xml:base

[Jonathan Kingston]

Since the deprecation of this feature the usage of this feature has reached
near 0 numbers in our usage counters[1]. 0.003% based on 64 beta [2]

The specification has removed xml:base many years ago and this change has
an advantage of code simplification and also reduces the need to compute
base paths in the DOM.

IE, Safari and Chrome don’t support this feature so we should remove this
feature from the web platform given it’s declining usage.

Mozilla attempted to remove this before[3] previously however the feed
reader appears to have been a blocker for it’s removal due to xml:base
usage within RSS feeds. However this has since been removed and no longer a
blocker.

Kind regards

Jonathan

[1]
https://telemetry.mozilla.org/new-pipeline/dist.html#!cumulative=0&end_date=2018-11-26&include_spill=0&keys=__none__!__none__!__none__&max_channel_version=nightly%252F65&measure=USE_COUNTER2_DEPRECATED_XMLBaseAttribute_DOCUMENT&min_channel_version=nightly%252F62&processType=*&product=Firefox&sanitize=1&sort_keys=submissions&start_date=2018-10-22&table=0&trim=1&use_submission_date=0

[2]
http://georgf.github.io/usecounters/#kind=page&group=DEPRECATED&channel=beta&version=64

[3]
https://groups.google.com/forum/#!topic/mozilla.dev.platform/1JDnJWefe1E/discussion

[Jonathan Kingston]

Sorry I forgot to link to the bug itself:
https://bugzilla.mozilla.org/show_bug.cgi?id=903372 which hasn't changed
from last time.