It's a wildly nontrivial problem if you're trying to only be forward moving and want to minimize your buffer.

That's why everybody else either rerenders (such as rich) or relies on the whole buffer (such as glow).

I didn't write Streamdown for fun - there are genuinely no suitable tools that did what I needed.

Also various models have various ideas of what markdown should be and coding against CommonMark doesn't get you there.

Then there's other things. You have to check individual character width and the language family type to do proper word wrap. I've seen a number of interesting tmux and alacritty bugs in doing multi language support

The only real break I do is I render h6 (######) as muted grey.

Compare:

    for i in $(seq 1 6); do 
      printf "%${i}sh${i}\n\n-----\n" | tr " " "#"; 
    done | pv -bqL 30 | sd -w 30

to swapping out `sd` with `glow`. You'll see glow's lag - waiting for that EOF is annoying.

Also try sd -b 0.4 or even -b 0.7,0.8,0.8 for a nice blue. It's a bit easier to configure than the usual catalog of themes that requires a compilation after modification like with pygments.