HTML Concepts: Customized Built-In Elements

Published on March 25, 2023 (↻ July 2, 2023), filed under (RSS feed for all categories).

HTML allows to define custom elements, elements which enable authors to “build their own fully-featured DOM elements.”

One special type of custom element is the customized built-in element… a custom element built on an existing HTML element. This allows for reuse and extension of functionality already available.

The way customized built-in elements work is essentially this:

  1. Pick an element to be built on and extended.

  2. Define a class for it, similar to an autonomous custom element (or as MDN Web Docs shows).

  3. Have that class extend the respective element (like HTMLButtonElement for a button element).

  4. For the define method, also specify an extends option (like extends: "button").

  5. To use the new customized built-in element, add an is attribute to the customized element in question, with the name of the customized element (like <button is="example-action">Action</button>).

It’s useful to know that to ensure future-compatibility, this only works for elements defined in the HTML specification. That is, currently undefined elements but also deprecated elements cannot be extended like this, which includes legacy elements like isindex, keygen, or nextid (sure, elements everyone has been waiting to customize!).

Yet in general, that’s it—though things become more interesting depending on the functionality to be added.

Customized built-in elements enjoy broad support, but are not currently supported by every browser. Follow the post thread on Mastodon for more details, or to share your information.

Have a look at other concepts—or review the summaries of the first eight concepts over at Frontend Dogma.

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About Me

Jens Oliver Meiert, on September 30, 2021.

I’m Jens, and I’m an engineering lead and author. I’ve worked as a technical lead for companies like Google and as an engineering manager for companies like Miro, I’m close to W3C and WHATWG, and I write and review books for O’Reilly and Frontend Dogma.

With my current move to Spain, I’m open to a new remote frontend leadership position. Feel free to review and refer my CV or LinkedIn profile.

I love trying things, not only in web development, but also in other areas like philosophy. Here on meiert.com I share some of my views and experiences.