Two years ago, I started the ProseMirror project because I wanted to take a stab at a better approach to WYSIWYG-style editing. Today, I'm releasing version 1.0 of the library. The architecture and scope of the project have changed quite a bit during its lifetime, but I feel that the original goal has been met.
Most importantly, ProseMirror is agnostic to the actual document shape, making it possible to build applications on top of this library that in the past would have required a fully custom editor implementation.
What I mean by being agnostic to document shape is ProseMirror's schema feature. The core editor has no built-in opinion about what a document looks like, and instead looks at a piece of configurable data (the schema) to figure out what kind of content is allowed and how it is structured. ProseMirror will work with precisely the custom semantic document format that you need, while still giving you the WYSIWYG style of editing that users are used to.
For example, a scientific writing app could use a schema that includes sections, footnotes, and references—two such apps, SciFlow and Fidus Writer have been built on top of ProseMirror. Or a news organization could build a schema that reflects their content model, to provide an editor for journalists to write in. For example, The New York Times is using ProseMirror in its CMS. Or if your company has editors for a number of differing content models, using ProseMirror with different schemas can make it easier to unify your editor code. Atlassian is rolling out ProseMirror across their products, ranging from wiki to bug tracker to source hosting.
Support for collaborative editing has been a focus in ProseMirror from the start. Several aspects of the system, such as the way document updates are represented, or the way the undo history module works, have been strongly influenced by the requirements of collaborative editing. I've become convinced that this is not a feature you can robustly bolt onto an existing rich text editor.
Fortunately, these constraints, rather than forcing the design into an uncomfortable corner, helped push it in a generally beneficial direction. Several other tricky applications, such as change tracking and the ability to roll back past changes, were made possible by design decisions made for collaborative editing-related reasons.
Trying to combine the requirements of collaborative editing with a functional unidirectional data flow architecture led us to a design where the editor, instead of unilaterally updating its state, emits transactions. A transaction can be used to compute a new state with which to update the editor.
This makes it possible to almost seamlessly integrate the editor in your application's data flow cycle if you want to. In addition, having updates as first-class values makes it much easier to keep external state in sync with the editor, which allows new, powerful types of extensions.
After years of wild experiments and constant change, starting with the 1.0 release we are aiming for stability. The central modules will stay on 1.x for as long as possible, which means new releases won't require you to change your code. There's an RFC process that we'll use to get community feedback on new features
If you're looking for a simple drop-in rich text editor component, ProseMirror is probably not what you need. (We do hope that such components will be built on top of it.) The library is optimized for demanding, highly-integrated use cases, at the cost of simplicity. But if your app is pushing the limits of what has been possible with WYSIWYG editors so far, take a look.