Per this aria-readonly on an element with contenteditable test, the ARIA validator is correctly indicating that aria-readonly is invalid, but not for the expected reason. (e.g., presently it just indicates that aria-readonly is invalid on a div, which would be sufficient if not for the contenteditable attribute)

There should be a rule that explicitly looks for if an element has contenteditable, and if so, throw an error if aria-readonly=true is also set to that element.