Mars Tomorrow is a terraforming strategy game played in the browser: you're given a section of Mars and the job of making it habitable, managing rovers, raising the atmospheric pressure, melting ice into water and eventually getting plants to grow.
It's slower and more scientific than most browser strategy games — changes to temperature, pressure and water interact, so it rewards understanding the system rather than clicking faster.
Deliberately changing a planet's atmosphere, temperature and surface to make it habitable. In the game that means raising pressure and temperature, creating liquid water, and then introducing vegetation.
Yes — it runs in the browser with no download, though it's a longer-form strategy game than most browser titles.
Yes — it runs in the browser with nothing to install.