Probably start with one that would create some clear Value for
stakeholders. For instance, an app, that would eventually be able to
tell us if more web users have more than 1 browser installed on his/
her computer.
This information would be important to many web app and site
developers.
Assuming that the OP wishes to run their project in a browser as a web
application (which is the presumption here unless stated otherwise),
it is unlikely that a javascript program will ever be able to do that.
And even if it could, it's pointless as the results don't tell "web
app and site developers" anything they can't learn from their server
logs about which browsers are actually being used to visit their site
(even allowing for the vaguaries of such statistics).
I have 6 browsers installed on the PC I'm using now. I use one 99.9%
of the time for web browsing, the others are for testing and
miscellaneous tasks.
On the plus side, it would take some creative thinking to find uses
for "most\all features of HTML5" in such an application if it *was*
possible.