Capability Tiers

The three surfaces differ not in how they present the same data, but in which data exists at all. A flat trait with Option return values would lead to rules silently not running depending on the host.

	static HTML	Chrome via CDP	In-page WASM
Document: structure, tags, attributes, text, hierarchy	✓	✓	✓
Semantics: role, accessible name	computed	native	computed
Rendering: styles, geometry	–	✓	✓
Interaction: focus, events	–	✓	✓

Every rule declares its Tier; every host implements the traits it can serve.

The boundary lies in the type system

Rules are function pointers, registered separately by tier: StructureRule<D: Document>, SemanticsRule<D: Semantics>, RenderingRule<D: Rendering>. A tier 2 rule cannot even be called with a host that does not satisfy Semantics. No trait objects: this monomorphizes per host and allocates nothing per rule.

Not run is not passed

The entry functions run with what the host provides and record the rest. run on a structure-only host lists every tier 2 and tier 3 ID as a RuleRun with NotRun::CapabilityMissing and a reason:

{
  "rule_id": "links/name-missing",
  "not_run": "capability_missing",
  "findings": 0,
  "reason": "host provides no role and no accessible name"
}

The excerpt comes from examples/teaser.struktur.json. Without this record, a contrast check without rendering access would read like a passed check.

The tree is DOM-shaped

Node models tags, attributes, text and hierarchy, not roles and accessible names. Most rules need attributes such as tabindex, id, role, alt or for, and the browser’s native accessibility tree does not provide them: tabindex does not appear there. Role and name are layered on top as a separate capability.

Requirement on tier 3

ComputedStyle::background_color is the effective background color, resolved across the ancestors. If the host cannot determine it, for example with a background image, gradient or background-blend-mode, it returns None, and the contrast rule reports contrast/text-undetermined with untested. A check against a guessed white would produce a pass that someone relies on.

Interaction is defined as a trait, but the 0.26.1 rule set has no rule on this tier yet.