Customized Builtins

[dandclark]

Description

Customized builtins allow developers to create customized elements that extend the behavior of native HTML elements.
https://html.spec.whatwg.org/multipage/custom-elements.html#custom-elements-customized-builtin-example

They are implemented in Blink and Gecko but not WebKit.

Tests:
https://wpt.fyi/results/custom-elements/builtin-coverage.html
https://wpt.fyi/results/custom-elements/customized-built-in-constructor-exceptions.html
https://wpt.fyi/results/custom-elements/parser
https://wpt.fyi/results/custom-elements/reactions/customized-builtins

Specification

https://html.spec.whatwg.org/multipage/custom-elements.html#customized-built-in-element

Additional Signals

• Many many developer comments advocating for these in various threads, e.g. The is="" attribute is confusing? Maybe we should encourage only ES6 class-based extension. WICG/webcomponents#509 and https://bugs.webkit.org/show_bug.cgi?id=182671, Customized built-in elements WebKit/standards-positions#97
• Multiple polyfills have been built for this, for example https://github.com/ungap/custom-elements-builtin (105 stars), https://github.com/corpusculejs/custom-builtin-elements, https://github.com/WebReflection/built-in-element
• WebComponents regularly show up as important to developers in surveys like the 2023 state of HTML survey where 42.9% of respondents had used WebComponents, or this MDN APIs & JavaScript survey where WebComponents were ranked as the top feature needing improvement.
• Many blog posts and articles have been written in favor of cusomized bultins. A few examples:
  • https://webreflection.medium.com/in-favor-of-custom-elements-built-ins-bae3f40f27d5
  • https://javascript.info/custom-elements#customized-built-in-elements
  • https://dev.to/steveblue/accessibility-with-customized-built-in-elements-2k8

[EisenbergEffect]

I thought we reached somewhat of a consensus at TPAC 2023 to work on custom attributes instead, since they would be able to do more than customized built-ins, without the issues that WebKit had raised.

[wpt-interop]

Thank you for proposing Customized Builtins for inclusion in Interop 2025.

We wanted to let you know that this proposal was not selected to be part of Interop this year.

On behalf of the entire Interop team, thank you for submitting this proposal for consideration. We got many more proposals than we could include in this year's project, necessitating some difficult choices. Please note this should not be taken as a comment on the technology as a whole, or our willingness to consider it for Interop in the future. We appreciate the work you put into your proposal, and would welcome your participation in future rounds of Interop.

For an overview of our process, see proposal selection. Thank you again for contributing to Interop 2025.

Posted on behalf of the Interop team.