Add support for also_requires_attr validator constraint

[westonruter]

Feature description

AMP requires that any element with an on event handler attribute also have a tabindex and a role=button. Currently, however, you can create AMP content in WordPress and validation errors will leak.

Consider a Custom HTML block containing:

<span on="tap:AMP.print">Print!</span>

Two validation errors leak to the frontend:

The reason for this the AMP plugin does not yet support the trigger like the on attribute spec has:

  attrs: {
    name: "on"
    trigger: {
      if_value_regex: "tap:.*"
      also_requires_attr: "role"
      also_requires_attr: "tabindex"
    }
  }

There are currently 22 instances of trigger in the protoascii, and each one is for also_requires_attr. This is currently called out as a todo:

amp-wp/includes/sanitizers/class-amp-tag-and-attribute-sanitizer.php

Lines 20 to 25 in 2549ca3

	* @todo Need to check the following items that are not yet checked by this sanitizer:
	*
	* - `also_requires_attr` - if one attribute is present, this requires another.
	* - `ChildTagSpec` - Places restrictions on the number and type of child tags.
	* - `if_value_regex` - if one attribute value matches, this places a restriction
	* on another attribute/value.

The tag-and-attribute sanitizer needs to be updated to support this constraint. This will include updating the Python spec parser to extract these values. When a failure is encountered a new validation error code should be raised. (Currently it is UNKNOWN in the AMP validator but we can do something specific.)

---

Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry