HTML Rules
Under the hood, most of the accessibility checks ran by Ace are powered by 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, taken from their documentation:
| Rule | Description |
|---|---|
| accesskeys | Ensures every accesskey attribute value is unique. |
| area-alt | Ensures <area> elements of image maps have alternate text. |
| aria-allowed-attr | Ensures ARIA attributes are allowed for an element's role. |
| aria-hidden-body | Ensures aria-hidden='true' is not present on the document body. |
| aria-required-attr | Ensures elements with ARIA roles have all required ARIA attributes. |
| aria-required-children | Ensures elements with an ARIA role that require child roles contain them. |
| aria-required-parent | Ensures elements with an ARIA role that require parent roles are contained by them. |
| aria-roles | Ensures all elements with a role attribute use a valid value. |
| aria-valid-attr-value | Ensures all ARIA attributes have valid values. |
| aria-valid-attr | Ensures attributes that begin with aria- are valid ARIA attributes. |
| audio-caption | Ensures <audio> elements have captions. |
| blink | Ensures <blink> elements are not used. |
| button-name | Ensures buttons have discernible text. |
| checkboxgroup | Ensures related <input type="checkbox"> elements have a group and that that group designation is consistent. |
| color-contrast | Ensures the contrast between foreground and background colors meets WCAG 2 AA contrast ratio thresholds. |
| definition-list | Ensures <dl> elements are structured correctly. |
| dlitem | Ensures <dt> and <dd> elements are contained by a <dl>. |
| document-title | Ensures each HTML document contains a non-empty <title> element. |
| duplicate-id | Ensures every id attribute value is unique. |
| empty-heading | Ensures headings have discernible text. |
| focus-order-semantics | Ensures elements in the focus order have an appropriate role. |
| frame-title-unique | Ensures <iframe> and <frame> elements contain a unique title attribute. |
| frame-title | Ensures <iframe> and <frame> elements contain a non-empty title attribute. |
| heading-order | Ensures the order of headings is semantically correct. |
| hidden-content | Informs users about hidden content. |
| html-has-lang | Ensures every HTML document has a lang attribute. |
| html-lang-valid | Ensures the lang attribute of the <html> element has a valid value. |
| image-alt | Ensures <img> elements have alternate text or a role of none or presentation. |
| image-redundant-alt | Ensure button and link text is not repeated as image alternative. |
| input-image-alt | Ensures <input type="image"> elements have alternate text. |
| label-title-only | Ensures that every form element is not solely labeled using the title or aria-describedby attributes. |
| label | Ensures every form element has a label. |
| landmark-main-is-top-level | The main landmark should not be contained in another landmark. |
| landmark-one-main | Ensures a navigation point to the primary content of the page. If the page contains iframes, each iframe should contain either no main landmarks or just one. |
| layout-table | Ensures presentational <table> elements do not use <th>, <caption> elements or the summary attribute. |
| link-in-text-block | Links can be distinguished without relying on color. |
| link-name | Ensures links have discernible text. |
| list | Ensures that lists are structured correctly. |
| listitem | Ensures <li> elements are used semantically. |
| marquee | Ensures <marquee> elements are not used. |
| meta-refresh | Ensures <meta http-equiv="refresh"> is not used. |
| 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. |
| object-alt | Ensures <object> elements have alternate text. |
| p-as-heading | Ensure p elements are not used to style headings. |
| radiogroup | Ensures related <input type="radio"> elements have a group and that the group designation is consistent. |
| region | Ensures all content is contained within a landmark region. |
| scope-attr-valid | Ensures the scope attribute is used correctly on tables. |
| server-side-image-map | Ensures that server-side image maps are not used. |
| 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. |
| table-fake-caption | Ensure that tables with a caption use the <caption> element. |
| td-has-header | Ensure that each non-empty data cell in a large table has one or more table headers. |
| td-headers-attr | Ensure that each cell in a table using the headers refers to another cell in that table. |
| th-has-data-cells | Ensure that each table header in a data table refers to data cells. |
| valid-lang | Ensures lang attributes have valid values. |
| video-caption | Ensures <video> elements have captions. |
| video-description | Ensures <video> elements have audio descriptions. |
Modifications
Certain aspects of HTML accessibility checking don’t apply to EPUBs, so Ace has modified aXe’s default ruleset slightly.
- Ace disables aXe’s
bypassrule. - Ace changes aXe’s
landmark-one-mainrule to remove thehas-no-more-than-one-maincheck.