Do you mean display flex? You can use those, with the caveat that the .col class, which you can use for flex: 1 layouts includes extra padding and doesn't exactly work like flex: 1, which I never got that to work inline. Instead, I found a different one you can use called media-body.
There is another CSS feature called media-query you can use to respond to window sizes, but I don't think that's supported in HTML templates either.