It’s not that Martijn Cuppens used User Agent sniffing, CSS hacks, or anything like that to make this quirk div. This is just a plain ol’ <div> using the outline property a la:

div { outline: inset 100px green; outline-offset: -125px; }

It looks different in different browsers because browsers literally render something differently in this strange situation.

I happened upon Reddit user spidermonk33’s comment in which they animated the offset to understand it a bit more. I took that idea and made this little video to show them behaving even weirder than the snapshots…

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