[tzs]

The specs are open standards, but do the specs specific enough to actually do something that would work in practice?

By "work in practice" I mean handle most sites that people actually visit and display them almost identically to the way Chrome and Firefox do. If your new browser doesn't do the same thing as Chrome and Firefox on those sites, the users are going to perceive it as your browser sucking.

If Chrome and Firefox deviate from the spec in some way, you will have to match that deviation.

If there is something not covered in the spec but that browser have to deal with (such as handling malformed HTML, which a lot of sites have), you will have to match what Chrome and Firefox do there.