This is an archived snapshot of W3C's public bugzilla bug tracker, decommissioned in April 2019. Please see the home page for more details.
States "Unknown pseudo-element or pseudo-class :-moz-read-only" However as you can see here, it is valid. It is like this for several valid Mozilla CSS attributes. https://developer.mozilla.org/en-US/docs/Web/CSS/:read-only