I have Okular with markdown support which happens to be broken for okularpart so I got plain text render until I configured file association embedding for markdown to prefer kmarkdownwebviewpart did I get what was expected. So if okularpart was working I'd never even need to use kmarkdownwebviewpart. Of course I prefer it since I imagine it is a bit lighter that okular.
Thanks for the report. But what is the bug about exactly? Having to configure which component to use for embedded viewing in the file association is something which needs to be done for any kparts plugin. This is nothing specific for the kmarkdownwebviewpart.
I monitor cgit.kde.org and planetkde.org and that is how I learned of this effort. I use Gentoo and it happens to have ebuilds for everything I needed to get going. The issue is that there is insufficient information about the requirements to use kmarkdownwebviewpart. Specifically, looking at https://cgit.kde.org/kmarkdownwebview.git/tree/README.md, I had no idea that ktexteditorpreviewplugin is required. I also had no clue that I needed to move kmarkdownwebviewpart to the top of the list in the embedded viewing configuration for files associations. Those two pieces of information would help anyone else wanting to try this out. Otherwise it works well.
I installed kmarkdownwebview first and thought why is it not working? I see some screenshots and also how do I get the preview button? So hopefully you can see how one could easily think that it just doesn't work yet.
I see. Based on your feedback I have now added a section "Using" to the README.md, hopefully that helps the next person some more. I left out an explicit hint to the ktexteditor preview plugin though for now, as that one is simply one of several KParts plugins-using software and completely independent from this Markdown software, at least by technology. Still undecided, I could list it as one of a few example users, like I also did in the release announcements. Might add later, need to revolve that in my head some more, as it comes at cost of more details in the README.md :) In any case, intention of your report should have arrived, so closing it already, thanks for feedback.