Every built-in element is :defined

[jdalton]

Summary

:defined asks the custom element registry about every candidate's tag name and requires an instanceof match, so it matches upgraded custom elements and nothing else. Every built-in element is defined as well, which means the selector currently returns almost nothing on an ordinary page.

Only a custom element can be undefined, and only until a definition exists and the element has been upgraded to it.

What it returns before and after

<div id=d1></div>
<my-thing id=mt></my-thing>
<button id=b1 is="fancy-btn">x</button>

Chromium matches d1 with :defined and skips the other two, since neither custom element has a definition yet. Before this change the engine matched none of the three, including the plain <div>. After customElements.define('my-thing', …) runs, both Chromium and the engine add mt.

Where browsers implement it, and why the test is ordered this way

Blink reads the element's custom element state and treats "uncustomized" and "custom" as defined, in element.h#L1201, which its own comment ties to the DOM definition. :defined is that and nothing else, in selector_checker.cc#L3139.

The helper checks the hyphen in the tag name first, because a hyphen is what makes a name a valid custom element name, and asks for the is attribute only when there is none. An ordinary element therefore costs one string scan and no host call, which matters because :defined matches nearly every element in a document.

References: the spec, the browser source, and what each part was reasoned from

This patch applies to master on its own. The sixteen in this series were checked by cherry-picking them onto master one after another, in this order and in reverse, and all sixteen land without a conflict.

• Spec: concept element defined — uncustomized and custom are the two states that count as defined.
• Spec: custom elements core concepts — what makes a name a custom element name.
• Chromium: element.h#L1201 — the state read, with the same spec link in its own comment.
• Chromium: selector_checker.cc#L3139 — :defined is that and nothing else.
• MDN: :defined.

[jdalton]

This one sits with #190 and #191 and #192, which touch the conformance fixes found by comparing against Chromium.

They do not depend on each other. All seventeen in the series cherry-pick onto master in any order, and I checked that in both directions, so any one of these can land alone. The order below is the one they read best in:

• #190 makes :enabled the complement of :disabled, fieldsets included.
• #191 anchors the :required and :optional tests, and lets :optional take a button.
• #192 makes a fieldset :valid when none of its controls is invalid.
• #193 makes every built-in element :defined.