Add is attribute for Custom Elements

[snuggs]

whatwg/html@e5dc6bf
web-platform-tests/wpt#9508
whatwg/html#3402
whatwg/dom#566

[GeorgeTailor]

Webkit safari devs stated that they would never implement is. So why bother with it?
stats show that almost 20% of users use Safari, either desktop or mobile.

[snuggs]

@GeorgeTailor have a ticket reference? I'd be MORE THAN HAPPY to close this issue. This is like watching paint dry.

Have asked the team to chime in.

Added you as a contributor to this project. Thanks!

[GeorgeTailor]

WICG/webcomponents#509 (comment)
That was quite a discussion with almost 500 comments xD

[snuggs]

@GeorgeTailor copy that! I think that's a good enough reference. I remember that long conversation. Let's close this out! Can always reopen it later. Feel free to skim through any other issues. I'm sure you have already xD.

/cc @rianby64