1. Easy
2. the same on each platform
3. do not involve scintilla 1000 functions and callbacks
4. support pictures, rich formatting and all that end- user mess, needed for pretty documents , emails and so on.
5. handle all possible 100+ character sets and UTF correctly (yes, in many countries they use national character sets still in web content and emails)
Solution which seem to work with PB just fine
Presentation and editing:
use ckeditor 4 on windows platforms and set IE emulation to IE 11 (no idea what happens after Feb. 203 when MS retires IE permanently)
use ckeditor 5 on other platforms (newer webkit is needed, search the forum)
create html files with ckeditor and insert your html file into the <editor> div.
Edit at will.
for Getting the sanitized and edited content:
use any of the java script callbacks (on Webkit - there are solutions on this forum, on IE - haven't yet found it, but using windows API does work in other languages, so it will work with PB)
Downsides:
- obviously terrible memory hog (RTF with HTML-RTF-HTML converter would be much much better for presentation and editing)
- lots of additional files to distribute and maintain. No more single executable

- switching ckeditor for different platforms and watching how it becomes obsolete or incompatible
Question:
Any chance for a unified rich-text editing platform like chromium or RTF (not "Webview" - this is is utterly Windows only) in the near future?