And that message should link to https://developer.mozilla.org/en-US/docs/Mozilla_Quirks_Mode_Behavior
Finding out if the document is in quirks mode seems as simple as checking that document.compatMode !== "CSS1Compat" is true. There's some code in Firefox that already uses this: the page info popup (shown when you select "show page info" in the page context menu).
Note that about:blank is in quirks mode (and has to be for Web compat, IIRC).
This Chrome DevTools issue is very similar: https://bugs.chromium.org/p/chromium/issues/detail?id=622660&q=component%3APlatform%3EDevTools%20layout&colspec=ID%20Owner%20Summary%20Modified%20Stars There's even a mockup for adding a message to the inspector attached.
Should this bug handled by the console component ? It looks like the warning message should be logged from the platform side.
Severity: normal → enhancement
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