Linux user here.. MXLinux.

I use Geany almost only as an html-editor.

I've been using an old version of Epiphany a.k.a. Web as the live preview, because it automatically updated each time a save was made in the source code in Geany. But it was not very stable and slowed down a lot. For the moment I use Firefox as the preview engine. It's okay, but there's no auto-refresh, so I have to hit the FF refresh button after each change.

In older  Geany information there was talk about a Geany html plugin that had a html preview. That plugin seems to be removed.

I found an unofficial package on the web, downloaded it, but there the joy ended: It seemed to be source code, needing compiling, something that is far beyond my knowledge.  It was called "webkit-preview" or something similar.

Hope I described my situation enough and hope to read how  you experts have solved this.

Gratefully!