[g1BSsQ]

Summary

BaseDocument::new enables Stylo prefs with style_config::set_pref! "before creating the Stylist", but the default user-agent stylesheet is parsed before that point and only once:

• UserAgentStylesheet::default_sheet() is a LazyLock around UserAgentStylesheet::parse(DEFAULT_CSS).
• DocumentConfig::default() calls default_sheet(), so the UA sheet is parsed before BaseDocument::new sets any pref.

As a result, every pref-gated declaration in assets/default.css is dropped at parse time, and it stays dropped for the life of the process. For example, ul, ol, menu { counter-reset: list-item; } is lost, because counter-reset/counter-increment are gated by layout.css.counters.enabled in the pinned Stylo. Author stylesheets are parsed later and are not affected, so the problem only shows up for UA rules.

Observed: the computed counter-reset of an <ol> was empty. When the prefs are set before the UA parse, it is list-item 0.

Fix

• Move the set_pref! block out of BaseDocument::new into init_stylo_prefs(), guarded by std::sync::Once. The prefs and values are unchanged, including the writing-mode feature-gated ones.
• Call it at the start of UserAgentStylesheet::parse, so the default sheet and any custom UA sheet are parsed with the prefs on.
• Also call it at the start of BaseDocument::new, so documents built without a UA sheet still get the prefs.

No behaviour change for author styles. No new dependencies.

Measured impact

WPT run (cargo run -rp wpt) on a fork CI, comparing this branch against main at fe33b77 on the same runner:

• Suites: css/css-lists, css/css-contain, css/css-writing-modes and html/rendering (2,690 tests).
• Result: 0 newly passing, 0 newly failing.
• The run was at 7caba8a. The later commit only removes a blank line for rustfmt.

That is expected: list markers do not read the list-item counter today, so the UA counter rules are now present but not yet consumed. This change is a prerequisite for CSS-counter-based list numbering, such as ol[reversed], <li value> and author counter-reset/counter-increment on list-item. Without it, that work would see the UA defaults missing.

WPT results

No changes in test results compared to main.

_{Generated by the WPT workflow.}