RFS is weird with non-fluid containers

Created by: alekseyg

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

It would be nice to have responsive font sizes that have "steps" to them which follow the media queries if your website uses any of the containers that essentially are fixed width at most screen sizes. I know we have fluid RFS, but this would be a setting specifically for non-fluid containers.

Motivation and context

I've seen people mention a lot of things close to this, but none of them really mention the particular weirdness of RFS and containers. .container-fluid or the larger ones that are fluid until you get to XL or XXL work beautifully with RFS, where both the container and the font size grow along with the viewport. If you're using .container or the various smaller ones which essentially are fixed width per range of viewport widths, then the growing and shrinking of the font sizes is weird and mostly negates the benefit of having fixed width containers. On the other hand, if you disable RFS completely, then the font sizes are quite large on mobile devices. It would be nice then to have several RFS modes: off, steps, and fluid.