A nice little affordance that you don't usually get in online editors but do get in text editors like VS Code, Sublime, and Atom. Highlighting text then pressing "*" or "_" inserts those characters around the selection. Sort of like command + i or command + b, but with the markdown symbol itself.
It was pretty difficult to implement, mainly because the range has to be reset after changing the underlying HTML, which means keeping track of the selected range before and after making the change, then reconciling the difference. There are still some things to fix.