It's not clear to me from the existing documentation exactly what types of certificate are appropriate for signing HTML5 CC2014 extensions.
I used a Thawte “Adobe Air” certificate for signing Flash-based extensions. This has now expired. Can I purchase the same type and will that work cross-platform for signing HTML5 extensions?
Certificate requirements have not changed for HTML5 extensions, so the same certificate type as you had before will work ("Adobe Air" certificates are fine).
Note that if you are only distributing your HTML5 extension via Adobe Add-ons / Adobe Exchange, then there is no need to use a paid-for certificate. If you are distributing your extension elsewhere, then you should do the same as you did with AIR extensions.