(1) could be answered with container query units. Set `container-type: inline-size;` on the parent element (falls back to the whole viewport if unset), then use, e.g., `width: 50cqw; height: 50cqh` in children. The value is a percentage of the given axis
I don't think that works if you want to mix "proportion of available space" units with siblings that have a fixed size (a super-common use case for flexbox - having a fixed size box followed by another box that takes up "the rest of the space").
