Intent to implement: transitionstart event

[Mantaroh Yoshinaga]

Summary:
CSS Transitions Level 1 defines the transitionend event but provides no
means for detecting when a new transition is triggered (unlike CSS
Animations which defines an animationstart event). As a result, after
applying changes to content (e.g. adding a new class), authors don't
know if it is necessary to wait for transitionend event or not--a
problem we ourselves sometimes encountered developing apps for Firefox OS.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1287983

Link to standard:
https://drafts.csswg.org/css-transitions-2/#eventdef-transitionevent-transitionstart

Although this event is only defined in a very early editor's draft of
CSS Transitions Level 2 (Level 1 is still a working draft) this event
has already shipped in Edge[1]. A bug is on file to implement in Blink
but has no commitment to implement yet (other than recently being added
to Hotlist-Squash-A-Bug).[2]

[1] https://msdn.microsoft.com/library/dn632683(v=vs.85).aspx
[2] https://bugs.chromium.org/p/chromium/issues/detail?id=439056

Platform coverage: all platforms.

Estimated or target release: 51

Preference behind which this will be implemented: Not needed.

DevTools bug: Not needed?

[Brian Birtles]

On 2016/08/22 8:18, Mantaroh Yoshinaga wrote:
> Although this event is only defined in a very early editor's draft of
> CSS Transitions Level 2 (Level 1 is still a working draft) this event
> has already shipped in Edge[1].

Unfortunately, it appears the spec behavior does not agree with the Edge
behavior[1] so we'll need to resolve that first.

[1] https://github.com/w3c/csswg-drafts/issues/416