Summary: `contentReadOnly` and `insertBrOnReturn` are available with `Document.execCommand`, `Document.queryCommandState`, etc, but they are Gecko specific legacy command. We've already disabled them in early beta and earlier builds since 4 years ago. Even though there are users of these commands, but we don't get any regression reports. Additionally, the effect of these commands are big and it's hard to use these commands in public editable web apps since making the builtin editor incompatible from the other browsers. So, it seems that they are used for fingerprinting.
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1986838
Platform coverage: All platforms.
Preferences: dom.document.edit_command.contentReadOnly.enabled
and dom.document.edit_command.insertBrOnReturn.enabled
DevTools bug: N/A
Other browsers: Nobody supports these Gecko specific commands.
-- Masayuki Nakano <masa...@d-toybox.com> Working on DOM, Events, editor and IME handling for Gecko