[mixmastamyk]

You make a repo with the same name as the your user account and domain, e.g. user.bitbucket.org

[Achshar]

I already have such a repo. But I would reckon the repo needs to be open source right? What if I would like to host all my open source repos on that subdomain?

Oh ok, it needs to be the entire url 'user.bitbucket.org'.

[mixmastamyk]

Don't believe there are restrictions, but only one repo resides at that url. Put in an index.html and it should be viewable by browser.

[joelg236]

It doesn't need to be open source, no. It will host anything in the repo as static content, even if the repo is private.