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Why does that feature even exist?

So dessigners can make adjustments depending on what fonts are available.

Is there any website actually using it for that purpose? Usual CI policies would be "actively disinterested" in it.

Any website with a rich text editor would use it.

AFAIK it's not really a feature it's more of an exploit.

Ultimately, the sizing of a font can effect element widths, and element widths can be queried.