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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. Ace uses axe’s rules, with some modifications (described here).

Rules

Here is a list of axe’s rules used by Ace (see the full up to date documentation in GitHub ):

Rule ID Description
accesskeys Ensures every accesskey attribute value is unique
aria-allowed-role Ensures role attribute has an appropriate value for the element
aria-dialog-name Ensures every ARIA dialog and alertdialog node has an accessible name
aria-text Ensures "role=text" is used on elements with no focusable descendants
aria-treeitem-name Ensures every ARIA treeitem node has an accessible name
empty-heading Ensures headings have discernible text
epub-type-has-matching-role Ensure the element has an ARIA role matching its epub:type
frame-tested Ensures <iframe> and <frame> elements contain the axe-core script
frame-title-unique Ensures <iframe> and <frame> elements contain a unique title attribute
heading-order Ensures the order of headings is semantically correct
identical-links-same-purpose Ensure that links with the same accessible name serve a similar purpose
image-redundant-alt Ensure image alternative is not repeated as text
label-title-only Ensures that every form element is not solely labeled using the title or aria-describedby attributes
landmark-banner-is-top-level Ensures the banner landmark is at top level
landmark-complementary-is-top-level Ensures the complementary landmark or aside is at top level
landmark-contentinfo-is-top-level Ensures the contentinfo landmark is at top level
landmark-main-is-top-level Ensures the main landmark is at top level
landmark-no-duplicate-banner Ensures the document has at most one banner landmark
landmark-no-duplicate-contentinfo Ensures the document has at most one contentinfo landmark
landmark-no-duplicate-main Ensures the document has at most one main landmark
landmark-one-main Ensures the document has a unique main landmark
landmark-unique Landmarks should have a unique role or role/label/title (i.e. accessible name) combination
meta-viewport-large Ensures <meta name="viewport"> can scale a significant amount
meta-viewport Ensures <meta name="viewport"> does not disable text scaling and zooming
page-has-heading-one Ensure that the page, or at least one of its frames contains a level-one heading
presentation-role-conflict Flags elements whose role is none or presentation and which cause the role conflict resolution to trigger.
region Ensures all page content is contained by landmarks
scope-attr-valid Ensures the scope attribute is used correctly on tables
skip-link Ensure all skip links have a focusable target
tabindex Ensures tabindex attribute values are not greater than 0
table-duplicate-name Ensure that tables do not have the same summary and caption

Developer note: regular expression replace all: (\| .+? \| .+? \|) .+? \| .+? \| .+? \| .+? \| => $1

Modifications

Certain aspects of HTML accessibility checking don’t apply to EPUBs, so Ace has modified axe’s default ruleset slightly.