Well, this is a Safari beta.

There's a flag in Chrome. It seems to have landed in early December. https://chromestatus.com/feature/6525308435955712

I know it might be year or more after this releases that this will be actually usable (due to adoption) but I cannot wait for this. I hate having to do math/calculations based on viewport height/width to decide how a container should behave at different sizes. There are just so many things that have to be taken into account and all your math/logic falls apart when you change almost anything else that affects the container.

As a Chrome apologist really this pisses me off but I've been waiting for this forever and applaud the team (especially if they beat Chrome to landing this in a stable release)