aria-readonly="true" is not allowed on an element with contenteditable #353
Description
Steps:
- go to https://w3c.github.io/html-aria/tests/contenteditable-readonly-test.html
- run accessibility checker
- the violation for "The WAI-ARIA role or attribute 'aria-readonly' is not valid for the element
<div>" is good, but there should also be a specific violation for aria-readonly="true" conflicting with contenteditable, as per https://w3c.github.io/html-aria/#att-contenteditable