>It's constraints like "line up the left side of widget A with the right side of widget B" that can be slow. In this case no width is provided for each widget, so the constraint solver has to find one (which likely involves calling into the widgets to size themselves, adjusting the sizes according to some algorithm and then laying out those widgets again with the final size).

this problem somewhat already exists with layout thrashing https://web.dev/articles/avoid-large-complex-layouts-and-lay...

And given how layout thrashing and similar problems work I feel that you can code CSS in a constraint manner at least part of the time.