Bench Virtual Rule Mirror Skill

bench/config.ts does not extends: ['markuplint:html-standard']. It curates rules explicitly so the bench surface stays scoped to nu-validator capability. Consequence: any new entry in a preset's nodeRules[] is invisible to the benchmark until the same selectors are mirrored into bench/config.ts. Symptom: fixtures the new virtual rule should detect remain at nu-only and look like a markuplint coverage gap when the rule already exists.

Procedure

1. Add the selector(s) to the relevant preset (preset.html-standard.jsonc or similar).
2. Mirror the same selectors into bench/config.ts nodeRules[] with a comment naming the source preset rule (e.g. // Mirrors html-standard/no-duplicate-charset).
3. Run yarn bench:update:ml.
4. Verify the target fixtures flipped from nu-only → match-error. If they didn't, the selectors don't actually match those fixtures — check selector syntax (caveats below).
5. If the rule does not map onto any nu-validator capability, skip step 2 and add a // not mirrored: <reason> comment to the preset rule's JSDoc so the next maintainer knows it was a conscious decision.

Selector caveats

• HTML attribute names are matched ASCII case-insensitively ([charset] matches <meta CHARSET>). Use lowercase to match HTML's normative form.
• The i flag (e.g. [name="description" i]) only applies to attribute values — it is unnecessary for the name, which is already case-folded for HTML.
• Sibling selectors (~) only match elements after the anchor. Coexistence-style constraints typically need both directions (a ~ b and b ~ a) so the rule fires regardless of source order.