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Custom Elements

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This page documents some of the discussion about Custom Elements in [email protected] from January to March 2015.

Rough consensus

There appears to be rough consensus for:

  • ES6-style classes
  • Subclassing of HTMLElement and SVGElement
  • registerElement() API to tie a class to a local name
  • Lifecycle callbacks for: insertion into a document, removal from a document, mutation of an attribute

Additional lifecycle callbacks

There's a proposal in various bugs to offer these lifecycle callbacks as well:

  • Adopting so when moving nodes across documents adjustments can be made (e.g. an <img> has its animation restarted). This callback needs to be passed the old and new document.
  • Cloning so cloneNode() can have similar semantics for custom elements as it does for e.g. <input>. For cloning we want the browser to create a clone (via the constructor) and copy all content attributes. Then the callback is passed the clone, a document, and the clone children flag (note that these are only needed for children not part of a tree, for elements such as <template>, children part of the tree are handled by the browser).

Use symbols to identify lifecycle callbacks

It has been suggested that we should use symbols rather than names ending in Callback to avoid collisions with libraries.


Upgrading is the concept of going from a piece of markup, such as <my-div data-teehee="💩"></my-div>, to an object in a tree.

Tactic Explanation Drawbacks
Brain transplant A basic element is created by the parser and a callback registered by the developer is invoked at a later point to turn that basic element into a custom element.
  • Not having identity at creation-time is a giant mismatch with the rest of the platform.
Dummy replacement A dummy element is created by the parser and replaced at a later point by an actual instance of the custom element created by invoking the constructor registered by the developer.
  • What about attributes on the dummy element?
  • What about event listeners on the dummy element?
  • What if the dummy element was removed from the tree (or moved around)?
  • Causes mutation observer "spam"
Synchronous constructor The constructor registered by the developer is invoked by the parser at the point the custom element is created and inserted into the tree.
  • Requires synchronization for document.write() for each custom element.
  • Nested event loop.
  • Does not work with asynchronously loaded assets.

(Written under the assumption that mutating the prototype of the basic element is no longer considered viable.)

Subclassing existing elements

Subclassing existing elements is hard as implementation-wise identity is both object-based and name / namespace based. Therefore subclassing an existing element (currently) requires that the name / namespace does not change.

A hack was invented to make this work: <button is="my-button">. That hack is not well liked leaving us two options:

  • We leave this for now and work on this in parallel while stabilizing a smaller subset of custom elements.
  • We block on this and delay even more.

(Assuming that not all implementers are suddenly going to be okay with this hack.)