I mean things as simple as bolding a piece of text. The way to get the browser to do this for you is to using the document.execCommand method, but the results are inconsistent between browsers. Worse than that though is stuff like hitting "enter" on the keyboard – all sorts of crazy stuff happens (new <span> tags being added, nothing consistent between browsers, generally doesn't do what you want). We came to the same solution the guys at Basecamp have: you generally can't trust the browser to get you from state A -> B and have to do it yourself.

Probably the hardest thing to do is handle copying and pasting, partly because most browsers give you very little control over this, so you have to resort to terrible hacks. You also have to decide how much of the formatting in the clipboard item was "intentional" and how much should be cleaned. I see if you copy/paste just a word from within Trix, it pastes it as a whole block with the block's formatting around it. I suspect this may surprise many users who expect it just to copy the word (the inline bit, not the block around it in editor parlance). Once you start going down this rabbit hole, you end up having to do things like take one DOM tree and recursively merge the "edges" with the adjacent trees to get it to behave as the user expects. I think we do a pretty decent job with Squire, but I'm sure there are still more edge cases we haven't covered.