Anchor the :required and :optional tests, and let :optional take a button

[jdalton]

Summary

The patterns behind :required and :optional are written /^input|select|textarea$/i, which does not anchor an alternation. A regular expression reads that as ^input or select or textarea$, so it accepts any element whose name starts with "input", any name containing "select", and any name ending in "textarea".

:optional also has to match <button> elements, which it currently skips.

What the loose pattern accepts, and the precedent for the fix

The alternation without a group means an element named inputmode matches ^input, and any custom element whose name contains select matches too. This file already writes the same kind of test correctly elsewhere — :empty and :placeholder-shown use /^(?:input|textarea)$/i — so this change makes the two form patterns match that existing style.

The same shape of bug was fixed in :link and :any-link, where /^a|area$/i accepted <abbr href="…">.

Why a button is optional, and where browsers say so

The HTML spec lists button elements first among the ones :optional matches, before inputs, selects and textareas. Blink answers true for a button without asking anything else, in html_button_element.h#L113, reached from selector_checker.cc#L2751.

A button has no required property, so !e.required is already true for one and adding it to the list is the whole change. button:optional returns the button afterwards, where it returned nothing before.

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: selector optional — the list of elements that match, which opens with button.
• Spec: selector required — and the list for the other half.
• Chromium: html_button_element.h#L113 — a button is optional outright.
• Chromium: selector_checker.cc#L2751 — how :optional is dispatched.
• MDN: :optional.

[jdalton]

This one sits with #190 and #192 and #193, 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.