The greatest mistake IMO is the way float state leaks out of blocks, as this is both extremely unintuitive and undesirable for performance reasons.[1] Floats should've been restricted to inline formatting contexts, with all in-flow blocks behaving as if they had `clear: both' set.

I also don't understand why they never specced the (much simpler) `text-align: -moz-left/-moz-right/-moz-center' which already had precedent in HTML with `<div align=left/right/center>'. It's the saddest part of the "center a div" saga, all the W3C had to do to fix it is to assign a standard keyword to a feature that everybody already implemented, but to this day it still hasn't happened.[2]

[1]: https://pcwalton.github.io/_posts/2014-02-25-revamped-parall...

[2]: After many long decades, they did finally specify block-level `justify-items'. Two problems: a) it's backwards-incompatible with text-align, b) it still doesn't work in Gecko.