Slate JS – A customizable framework for building rich text editors
I feel like some of the biggest difficulties Rich text editors in browsers face nowadays are supporting social media type inputs like mentions, hashtags, emojis, etc. while still maintaining the classic rich text selections. DraftJs is the only one that achieved that for me, I had to do some serious elbow grease to get functioning hashtags in Redactor (another rich text editor). 
SlateJS user mentions are far away from what users expect a mentions plugin to work like.