Ah, the joys of contenteditable HTML elements... Such an under-specified and utterly horrible part of the web specs.

I'm sure you had a lot of fun fighting Safari in general, and also the diverging handling between Chromium and Firefox (for example when pressing the Enter key). Even just changing certain CSS properties on the root editor element will cause the resulting markup on certain key inputs to be different.

I admire that you pulled it off, and hope that you have a big collection of cross-browser integration tests for catching all the regressions that will inevitably happen :) Good luck!