Hello, just making this issue to track the possible implementation (or possible desired lack-of-implementation) of customized builtins and the is="" attribute.
Ryosuke mentioned at https://github.com/w3c/webcomponents/issues/509#issuecomment-222860736 that Safari/Webkit would not implement is="". I'm against it.
I'm just opening this issue so that if/when Safari/Webkit decides to change their minds about is="" (I hope not) that we can be notified here.
(if I missed another issue, please link me there.)
P.S., I implemented an idea (in JS), "Element Behaviors", which is an alternative to is="" without some of the problems: https://github.com/w3c/webcomponents/issues/727.
*** Bug 187849 has been marked as a duplicate of this bug. ***
*** Bug 195403 has been marked as a duplicate of this bug. ***
As soon as Edge will ship on Chromium everywhere, WebKit will be the only browser not supporting this part of the living standard.
The popularity of the related polyfill is indeed increasing https://www.npmjs.com/package/@ungap/custom-elements-builtin and there are more and more projects directly or indirectly dependent on such polyfill.
The polyfill is 100% based on Custom Elements V1 and other Web standards, and it provides a great experience in WebKit/Safari without penalizing performance in a relevant way.
There are also a variety of posts from Web developers;community where the #1 complain is the "inability to extend builtin elements", which not only is false, since once polyfilled WebKit you cam do that alredy everywhere, but there are also various libraries using Server Side Rendering with builtin extends as a key feature.
AFAIK, the WebKit team has been always quite pragmatic in implementing features everyone uses anyway, so that polyfills won't be forever needed, hence I wonder if there will ever be the possibility to reconsider the WONTFIX, giving the feature has been desired for long time, but never landed natively in WK/Safari.
The inability of this browser to extend builtin is also feature-detectable, meaning it's easy to "penalize" WK/Safari after such feature detection, but in an ideal world, even not so ergonomic/elegant specs should be available cross browser, so that developers get a choice, and such choice comes from the platform.
As summary, please re-consider the current WK/Safari position, and provide a consistent cross-browser implementation of something voiced over and over as desired from Web developers.
Re-iterating on a possible V2 is always an option, but Custom Elements can be fully used already today, so it'd be super nice to have all the features natively available everywhere.