This makes it sound easy: after all, there aren't many browsers, but really the problem is that everything that talks SSL needs to support it. Browsers are the starting point, after that I guess the OS-level libraries, then everything else.
No. It would be nice if backend TLS tools supported TACK, but they don't need to for TACK to be hugely valuable, both by increasing the security of browsers themselves and by drastically expanding the surveillance network we have to detect bogus certificates.
This makes it sound easy: after all, there aren't many browsers, but really the problem is that everything that talks SSL needs to support it. Browsers are the starting point, after that I guess the OS-level libraries, then everything else.