Technically, Windows 8 is already very HTML5 compliant/based (+ WebGL in 8.1, faster than Chrome in tests so far from the OS / IE11), the whole metro interface and *I think* most of the store apps are based on HTML5 too.
However, it would be awesome to see a light-weight OS like that which isn't made by a huge corporation, would it also be possible to support the current/older web multimedia platforms too (Java, Flash, Unity Web Player, etc)?