HTML Rules

Under the hood, most of the accessibility checks ran by Ace are powered by Deque axe, an engine for automated testing of HTML.

Deque axe defines over 100 rules (including a few experimental and deprecated ones): https://github.com/daisy/axe-core/blob/v4.7.2_DAISY/doc/rule-descriptions.md

Note that the link above references the DAISY fork of Axe which adds a few EPUB-specific rules not present in Deque’s original ruleset (epub-type-has-matching-role and pagebreak-label). Also note that Ace defines additional EPUB rules related to publication package metadata, etc. (in other words, accessibility checks that require analysing other files than the publication’s HTML documents)