I don't trust UnderSteem with my password, can I still use this service some other way? [A: use posting key]
I strongly recommend this and also consider changing your posting key before and after using this site. You by definition do not know who is operating it, you do not know how good their security precautions are, and unless you are prepared to check the javascript for anything malicious every single time you use the site your key could be stolen and used to post and vote in your name, including things you very much would not like.
Good service @someguy123, I support what you are doing, but people also need to be careful.
Sadly there's not many options out there. We have STEEM Pressure which seemed like a good idea, but got nowhere. @fabien was also working on something and it's still not released. Now @natebrune is working on an extension to solve this issue.
All of those services (though perhaps not a browser extension) will likely involve some sort of connection to the clearnet, which would compromise privacy if people used them. I think what you are doing is the right approach, I'm not criticizing at all, just pointing out the risks to users and how to best minimize them.