Intent to implement and ship: Resize Observer API

[Boris Chiou]

Hi,

*Summary*:

We had landed this feature behind a preference last month, and we are
planning to ship this on Firefox 69. The ResizeObserver API is an interface
for observing changes to the element’s size (based on its content-box or
border-box, for now). Each time when the element's size changes, the
callback function would be triggered, and you can get the current
content-box size or border-box size information (or other boxes in the
future).

*Bug*: https://bugzilla.mozilla.org/show_bug.cgi?id=1543839
*Link to standard*: https://drafts.csswg.org/resize-observer-1/
*Platform coverage*: On all platforms.
*Estimated or target release*: Firefox 69
*Preference:* layout.css.resizeobserver.enabled

*Do other browser engines implement this?*

-
* Google Chrome: Supported since Chrome 64
<https://caniuse.com/#search=resize%20observer> *
-
* Safari: Will be available in Safari 13,
<https://bugs.webkit.org/show_bug.cgi?id=157743>(Webkit Bug 157743 has been
fixed) *

*web-platform-tests*:
https://searchfox.org/mozilla-central/source/testing/web-platform/tests/resize-observer
(or https://github.com/web-platform-tests/wpt/tree/master/resize-observer)

*Is this feature restricted to secure contexts?* No

If you have any concerns or questions, please feel free to let me know.
Thanks.

Regards,
Boris Chiou