This IDE extension is targetting RAD Studio XE version (ie. BDS 15.0 released in August 2010) specifically. It can hopefully serve as a work-around for the problem that the official Project Page IDE functionality mysteriously disappeared in that particular release, reappearing in later releases again.
It's implemented as a design-only package which can be simply installed in the IDE in the usual way.
From README.md, here's what it does:
This extension listens for the active project change events. If the currently activated project contains an HTML file with the name 'index.htm' or 'index.html' it will open and show it in the IDE's internal embedded browser.
A small demonstration: