Developing Axe-core Rules

Before you start writing axe-core rules, be sure to create a proposal for them in a github issue. Read Proposing Axe-core rules for details.

A rule is a JSON Object that defines a test for axe-core to run. At a high level, think of a rule as doing two things. First it finds all elements that it should test, and after that it runs a number of checks to see if those selected elements pass or fail the rule.

Rule Select and Matches

Each rule has a selector and optionally a matches property. The selector is a CSS selector. Each element matching this selector will be tested by the rule, unless the matches function says otherwise. The matches property is a reference to a function that returns a boolean, which indicates if the element should be tested.

The last thing that may influence if an element is selected for testing in the rule is it's visibility. By default, hidden elements are ignored by the rule, unless the excludeHidden is set to 'false'.

Using Checks in Rules

The actual testing of elements in axe-core is done by checks. A rule has one or more checks which end up generating a result. There are three properties with which to define a rule's checks. Each of them deals differently:

• All: Takes an array of check names, all of which has to return true for the rule to pass.
• none: Takes an array of check names, none of which can to return true for the rule to pass.
• any: Takes an array of check names, at least one of which has to return true for the rule to pass.

Rule Properties

Prop. Name	Description
id	Unique identifier for the rule
selector	CSS Selector that matches elements to test
matches	Function to further filter the outcome of the selector
excludeHidden	Should hidden elements be excluded
all	Checks that must all return true
any	Checks of which at least one must return true
none	Checks that must all return false
pageLevel	Should the rule only run on the main window
enabled	Does the rule run by default
tags	Grouping for the rule, such as wcag2a, best-practice
metadata.description	Description of what a rule does
metadata.help	Short description of a violation, used in the axe extension sidebar

Check Properties

Prop. Name	Description
id	Unique identifier for the check
evaluate	Evaluating function, returning a boolean value
options	Configurable value for the check
after	Cleanup function, run after check is done
metadata impact	"minor", "serious", "critical"
metadata.messages.pass	Describes why the check passed
metadata.messages.fail	Describes why the check failed
metadata.messages.incomplete	Describes why the check didn’t complete

Incomplete results occur when axe-core can’t produce a clear pass or fail result, giving users the opportunity to review it manually. Incomplete messages can take the form of a string, or an object with arbitrary keys matching the data returned from the check.

A pass message is required, while fail and incomplete are dependent on the check result.

Incomplete message string

As one example, the audio and video caption checks return an incomplete string:

messages: {
  pass: 'Why the check passed',
  fail: 'Why the check failed',
  incomplete: 'Why the check returned undefined'
}

Incomplete message object with missingData

As another example, the color-contrast check returns missingData to aid in remediation. Here’s the message format:

messages: {
  pass: 'Why the check passed',
  fail: 'Why the check failed',
  incomplete: {
    bgImage: 'The background color could not be determined due to a background image',
    default: 'fallback string'
  }
}

To wire up an incomplete message with a specific reason it returned undefined, the check needs matching data. Otherwise, it will fall back to the default message. Reasons are arbitrary for the check (such as 'bgImage') but they must match the data returned:

this.data({
    missingData: 'bgImage'
});

Hierarchical rules

Axe-core handles shadow DOM and cross-domain iframe rules very well - as long as you take care in the design and implementation of your rule. Any rule that evaluates the DOM hierarchy needs to think about what happens across shadow DOM boundaries and iframe boundaries.

The rule callbacks all receive both a node and a virtualNode argument (in addition to the options argument). node points to the DOM Node that is to be evaluated, whereas virtualNode points to the node in the flattened tree (the hierarchy that shadow DOM creates that is used for parent child relationships across shadow DOM boundaries).

If your rule looks at any hierarchical context (parents or children) then you need to operate on the virtualNode for those operations. Calls to isHidden and the accessible name calculation calls will all evaluate the hierarchy. The commons and utils functions will all fetch the virtualNode from the flattened tree if you use the node implementation (and then simply call the virtualNode one) and are there for backwards compatibility. This backwards compatibility comes at a performance cost that can be avoided by simply using the virtualNode to start with. We will ask you to change this during PR review, so you might as well just start out by using the virtualNode.

iframes

Rules that evaluate the structure and/or the number of elements on the entire page (for example the heading nesting rule, or the landmark rules) will need to do this evaluation across iframe boundaries. What this means is that the check function instead of determining pass/fail/incomplete, performs a data gathering function. This data is then passed up the iframe hierarchy to the top window where it is passed into the after function, which does the evaluation of the gathered data and determines pass/fail.

Rules of Thumb for Rule Code Reviewers

Rules of thumb for determining whether virtualNode should be used - if any of the answers to the following questions is yes, then it should use shadow DOM and virtualNode:

1. Is it using the non-virtualNode version of a function that wraps the flattened tree lookup?
2. Is it evaluating the DOM hierarchy?

Rules that do this MUST have shadow DOM test cases that show passes and fails across the shadow DOM boundary.

Rules of thumb for determining whether an after function is required - if any of the answers to the following questions is yes:

1. Does the rule evaluate number of things on a page?
2. Does the rule evaluate hierarchy of things across a whole page?

Rules that use an after function MUST have iframe test cases that assert correct data passing between iframes and handle all the relevant cases across iframes.