Not sure it is "plainly anticompetitive" in the legal sense. In the US, the laws on anti-competitive practices are the Sherman Act and the Clayton act. To be anticompetitive, the courts use the "per se" rule and the "rule of reason". "per se" rule covers things which are specifically listed in the laws as being anticompetitive (eg price fixing).

This isn't in the list of per se anticompetitive practises so it would need to be covered by the "rule of reason". That would require someone to demonstrate actual harm to competition that flowed directly from the illegal nature of the practise and was not compensated by some offsetting procompetitive benefit and there is no less restrictive alternative.

I don't see how a CSS property would meet the standard of actual harm to competition, especially since noone is stopping you from making your own liquid glass css if you want to (as far as I can see).