A fieldset is :valid when none of its controls is invalid

[jdalton]

Summary

:valid decides a <fieldset> by asking whether it contains a :valid descendant. The rule is the other way round: a fieldset is valid when none of its controls is invalid, which is a different question whenever a fieldset holds no validation candidates at all.

Such a fieldset currently matches neither :valid nor :invalid, so it falls out of both halves of a stylesheet that uses them.

The two shapes where the difference shows, and what browsers answer

<fieldset id=fsempty></fieldset>
<fieldset id=fs disabled>
  <input id=fi1>
  <fieldset id=fsin><input id=fi3></fieldset>
</fieldset>

fsempty has no controls, and fsin sits inside a disabled fieldset, so every control under it is barred from constraint validation. Chromium matches both with :valid. Before this change the engine matched neither, because neither contains a control that is itself :valid.

Where browsers implement it

Blink answers true for a fieldset's validity pseudo-classes and then loops its controls, failing only on one that is a candidate and invalid, in html_field_set_element.cc#L108. :valid is that pair of checks together, in selector_checker.cc#L2811.

The spec text is :valid, which says a fieldset matches when all of its descendant controls satisfy their constraints — a condition an empty fieldset meets.

The change is one character of logic: the engine already had s.first() and already compiled a :invalid resolver, so asking for the absence of an invalid descendant costs nothing new.

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 valid — a fieldset matches when all of its controls satisfy their constraints.
• Chromium: html_field_set_element.cc#L108 — the loop that fails only on a candidate that is invalid.
• Chromium: selector_checker.cc#L2811 — :valid as the pair of checks.
• MDN: :valid.

[jdalton]

This one sits with #190 and #191 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.