Escape sequences to change font features

[sh1boot]

Fonts support optional FourCC feature tags with standard names, like onum to switch to old-style numerals, tnum for "tabular" numbers (you should use this in the CSS for tables on web pages), and calt to enable ligatures (this is on by default) or smcp for small caps. Many of these would be useless on a monospaced terminal, but others could be useful for things like syntax highlighting. In addition, fonts can include their own ad-hoc tags. So you could put pretty much any feature you like in there. Text where the characters follow a curvy line, or have drop shadows, or are made out of bones, or whatever else you feel like squeezing into your font.

As things stand you can configure these tags in the font configuration for all terminal sessions, but you can't configure it for parts of the text in one window.

So I'd like to raise the possibility of a pair of escape sequences which wrap an arbitrary FourCC to enable or disable the corresponding font feature.

I'm going to guess that this may be costly, in that it opens up a huge number of permutations of configuration for every character on the screen, but there's probably a way to optimise that. I think what's more important is whether or not people would want to use it.