As per my recent "Intent to ship: Web Animations core interfaces"[1], we do
not intend to ship Animation composite modes until various spec work has
been completed.
However, when we implemented this feature, although we implemented it
behind a pref, we failed to turn off all the ways of accessing it.
Composite modes may be set in two ways:
1. Globally
e.g. elem.animate([{ opacity: 0 }, { opacity: 1 }], { duration: 2000,
composite: 'add' });
2. Per-keyframe:
e.g. elem.animate([{ opacity: 0, composite: 'add' }, { opacity: 1 }], {
duration: 2000 });
It turns out we successfully pref'ed off the feature when set globally (1,
from above) and even have automated tests to cover this, but we did not
pref off the feature when setting it per-keyframe (2).
No other UA is shipping this feature yet. As a result we don't have any
telemetry data regarding usage. However, given that no other UA ships this,
that it is not well known, and that setting the value on individual
keyframes is generally less convenient than setting it globally, I expect
this is not used at all outside of toy API demos.
Furthermore, by unshipping this Firefox will simply match the behavior of
current Chrome (that is, any animation using the feature would still run,
but would not show the additive behavior).
I think it is best to unship this now until it is properly specified and
then ship both modes together. As a result, in bug 1471814, which reached
m-c just moments ago, I have gone ahead and guarded this feature behind the
dom.animations-api.compositing.enabled pref (enabled in nightly only).
Please let me know if you have any concerns.
Best regards,
Brian
[1] https://groups.google.com/forum/#!topic/mozilla.dev.platform/fcFctnUjs7A
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