Intent to implement and ship: transitionrun, transitionstart, and,transitioncancel events

[Mantaroh Yoshinaga]

This is a follow-up to my previous intent to implement[1].

It was discovered that the implementation of transitionstart in
IE10~11/Edge differs from the specification so the specification was
subsequently updated to rename transitionstart to transitionrun and add
IE/Edge's transitionstart event.

This updated "intent to implement" is also expanded to cover the
transitioncancel event and includes an intent to ship as well.

Summary: CSS Transitions Level 1 now includes additional transitionrun,
transitionstart, and transitioncancel events to allow an author to
detect when a transition has been created when it begins animating, and
if it was terminated without running to completion. Authors often wait
for transitions to complete before updating UI state and these events
allow authors to know which transitions they need to wait for and
whether they should continue waiting (transitions may be prematurely
stopped for a number of reasons such as DOM nodes being detached from
the document tree--something that is particularly common with frameworks
such as Backbone and React).

Bugs:
https://bugzilla.mozilla.org/show_bug.cgi?id=1287983
(transitionstart/transitionrun)
https://bugzilla.mozilla.org/show_bug.cgi?id=1264125 (transitioncancel)

Link to standard:
https://drafts.csswg.org/css-transitions/#event-transitionevent

Platform coverage: all platforms.

Estimated or target release: 52

Preference behind which this will be implemented: Not needed.

DevTools bug: Not needed?

Do other browser engines implement this?
* IE10~11/Edge: Shipped transitionstart
* Blink: Considering? Bugs on file for implementing transitionstart[2]
and transitioncancel[3] have been given the "Hotlist-Squash-A-Bug" and
"Hotlist-Interop" flags respectively.
* The CSS WG resolved at TPAC this year to move these events from CSS
Transitions Level 2 to Level 1 to indicate that they are shippable.[4]

Tests: We have been writing tests for these using testharness.js (Web
platform tests) format. However, we don't intend to upstream these to
web-platform-tests immediately since (a) they rely on the Web Animations
API and (b) merging the CSS WG test repository with web-platform-tests
is a work in progress.

[1] https://groups.google.com/forum/#!topic/mozilla.dev.platform/PR_AP6i-xIo
[2] https://bugs.chromium.org/p/chromium/issues/detail?id=439056
[3] https://bugs.chromium.org/p/chromium/issues/detail?id=642487
[4] https://logs.csswg.org/irc.w3.org/css/2016-09-19/#e722575