FreeCiv is not HTML5. The game itself is implemented as a server, and you can write clients for it. I only know of a GTK+ client and a SDL client. Is there a HTML5 client around now?

"Freeciv.net is an open-source turn-based strategy game. It's built entirely in HTML5 and features in-depth game-play and a wide variety of game modes and options. Your goal is to build cities, collect resources, organize your government, and build an army, with the ultimate goal of creating the best empire.

Freeciv.net is an open source project, released under the GNU Affero General Public License 3.0 or later. The project is a fork of the C server from Freeciv.org, which has origins back to 1996. The idea of creating a version for Freeciv which would be playable online in a browser came about some time in 2007, and this project has evolved into what it is now since then. The first public beta release of Freeciv.net was available in December 2009, and the code is still under continuous development."