axe-core rule description

Hi team,
One of my actions from last week was to write up a description of rules in
axe-core, to compare it to other structures that are out there. So here
goes:

# Axe-core Rule Structure

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 has 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

- 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
- description:  Description of what a rule does
- help:  Description of how to resolve an issue
- all:  Checks that must all return true
- any:  Checks that must all return false
- none:  Checks of which at least one must return true

## Check Properties

- id:  Unique identifier for the check
- evaluate:  Evaluating function, returning a boolean value
- metadata impact:  "minor", "serious", "critical"
- metadata pass:  Describes why the check passed
- metadata fail:  Describes why the check failed


-- 
*Wilco Fiers* - Senior Accessibility Engineer