Implement what's missing to Flexbox Layout in order to support Bootstraps responsive Grid System

With flexbox layout we can take advantage of its wrapping "wrap" mechanism to make our support for responsive layouts better.

Bootstrap (.js) have a popular way of using flexbox layouts to make their UI's responsive with their Grid system [1].

However, Bootstrap relies on the fact that you can assign percent sizes to a child element of a layout. The percent is relative to the layout dimensions. We don't support that, because e.g. "width" accepts just a scalar number.

Figure out if there are alternative ways we can make the similar responsive layouts with the minimum API additions required.

Links & resources

1. Bootstraps Grid system: https://getbootstrap.com/docs/5.0/layout/grid/
2. chania.html demonstrates how this is done by bootstrap (but without bootstrap dependency)