I'm surprised to see no mention of the Trix editor here amongst the comments about how contentEditable is the root of all evil. Trix has a nice solution, from their readme:

  Trix sidesteps these inconsistencies by 
  treating contenteditable as an I/O devi-
  ice: when input makes its way to the ed-
  itor, Trix converts that input into an 
  editing operation on its internal docum-
  ent model, then re-renders that document
  back into the editor. This gives Trix 
  complete control over what happens after
  every keystroke, and avoids the need to 
  use execCommand at all.

Trix is from basecamp, and has a super healthy plugin/extension ecosystem - https://github.com/basecamp/trix

Full disclosure: not affiliated at all, but the rare times I need a WYSIWYG editor I tend to look for Trix and be very pleased.