X11 is stable and maintained but not getting new features. It will generally work well for most people but over time it will and is drifting to obsolesence.
Wayland has some flaws but is not basically stable and feature rich enough for most people to use. It is not a complete drop in for X11 and won’t necessairly ever will be but for the vast majorory of desktop users it is.
The problem with Wayland is that there are still issues for people with graphics drivers. Nvidia in particular has had serious issues with it although they are improving.
I personally still use X11 with my KDE set up because i still have problems woth Wayland. Thwyre not as bad as they were but its still not quite stable above for me.






I believe it does have HTML preview when using the document preview plugin and with kparts packages installed. When ive used the preview tool i think you can have a split screen view with one half the file and the other the preview, but ive not tested it with html.
I like Kate myself, its a good editor with support for numerous languages, and a decent session system and a range of plugins.