Boring demos are available anywhere you care to search. https://playcanvas.com/explore is the typical goto for this kind of 3d market these days. Particularly popular in the mobile OS and web game (e.g. facebook) space. Robostorm, seemore, and After The Flood are their demos.

Unity and Unreal can both export large games to the Web just fine but people found out they didn't want to serve 50 GB games to a temporary client.