Intent to implement: Custom Elements

[Edgar Chen]

Summary:
This is basically an after the fact notification that we're in progress of
implementing Custom Elements (both autonomous custom elements and
customized built-in elements) and the implementation for old spec, which
was never exposed to the web, will be removed. We are close to finishing
the implementation, but there are still some performance works before
shipping the feature. We plan to enable it *only* on Nightly first to get
more feedback from users. (there will be a intent-to-ship before shipping
the feature)

Tracking bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=889230

Spec:
https://html.spec.whatwg.org/multipage/custom-elements.html

Estimated target release:
59

Preference behind which this will be implemented:
dom.webcomponents.customelements.enabled

DevTools support:
TBD

Support by other browser engines:
- Blink: Partial support; supports "Autonomous custom elements" but not
"Customized built-in elements" [1]
- Edge: Under consideration [2]
- Webkit: Partial support; supports "Autonomous custom elements" but not
"Customized built-in elements" [3]

Testing:
We have web-platform tests for this; https://w3c-test.org/custom-elements/

BR,
Edgar Chen

[1] The customized built-in elements for Blink is under development;
https://bugs.chromium.org/p/chromium/issues/detail?id=648828&desc=4
[2]
https://developer.microsoft.com/en-us/microsoft-edge/platform/status/customelements/
[3] Safari has indicated that they will not implement customized built-in
elements;
https://github.com/w3c/webcomponents/issues/544#issuecomment-239301184