All the web platform changes are really cool:

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The CSS attr() function now supports values of any type, so authors can use attribute values in any property rather than only as strings in the content property.

Firefox now supports the progress() CSS function.

Firefox now supports the alpha() CSS function. This is a relative color function that can be used to modify just the alpha component of a given color.

Firefox now supports the Promise.allKeyed and Promise.allSettledKeyed methods from the await dictionary proposal, which take an object of promises and resolve to an object of results with the same keys.

... And so on.