Intent to implement: CSS Containment

[Kyle Zentner]

This proposed feature would allow authors to annotate when an element and
its descendants will not (or should not) affect the rest of the page. The
primary intended benefit of such annotations is to allow browsers various
layout and rendering optimizations.

As currently described <http://dev.w3.org/csswg/css-containment/>, this
feature adds a css property 'contain' with four potential values 'layout',
'style', and 'paint' as well as 'strict', which is equivalent to the
previous three. Details are still being worked out, as can be seen from the
mailing list.
<https://lists.w3.org/Archives/Public/www-style/2015Jun/0114.html>

This feature is tracked by Bug 1150081.
<https://bugzilla.mozilla.org/show_bug.cgi?id=1150081>

Most likely, this will be implemented in FF42, gated behind
"layout.css.contain.enabled". The timeline for performing the possible
optimizations will probably be a fair bit longer.

As far as I can tell, no other browsers are actively working on this,
although patches for 'contain: strict' were posted to blink-dev
<https://groups.google.com/a/chromium.org/d/msg/blink-dev/zAMZjAbxVJE/rDPK1jjwQigJ>
.