For security reasons, HTML files are not rendered in the web interface of Nextcloud, instead the plain HTML source code is displayed. In order to render a POSHDOC — or any other HTML file — a Nextcloud app is required.
The Nextcloud POSHDOC app could probably best be derived from an existing simple online editor application, for example the Markdown Editor.
Nextcloud applications are written in PHP and must be installed on the server.
When opening a HTML file with the Nextcloud POSHDOC app it is poshified, provided with a preconfigured style sheet and displayed. The app allows to switch to edit mode and to save the changes back to the original file on the Nextcloud storage