> Do you have any particular ones in mind. I guess that at the moment I'm
> most afraid of losing `four_space_rule` since it is so to speak a
> compatibility extension. OTOH maybe if I consistently use steps of 4 in my
> source (which I already do, e.g. 3 spaces after a list bullet) I won't
> notice much difference.

I think that's right, except in unusual cases (e.g., list numbers
greater than two digits).

999. hello
    four-space rule says that content starts here

999. hello
     commonmark says it starts here.