alt Hacker NewsWell, hang on, there are several different layout algorithms in CSS, right? They keep the old ones for backwards compatibility, and they have to interoperate which makes things more complicated; but flexbox and grid are relatively new and quite nice.
The people saying CSS is unfit for UI have probably never actually built a good UI in anything, and also realistically don't even know how to use CSS.
Flexbox on its own is a simple and very capable algorithm that you can use anywhere. You can do a lot with flexbox alone (see Yoga: www.yogalayout.dev) so it's a great place to start. Taffy supports a few more layouts (block, grid) which have their own separate use cases but flexbox is enough for most things.