I see no reason why tags like those should be ineligible. The whole idea behind "tag sponsorship" is that the sponsor has committed to purchasing a dominant portion of the advertising space for questions associated with that tag, and is then given the opportunity to be recognized for that and provide the community with some interesting/relevant links relating to the tag topic.
Obviously, I don't know every single company and product out there, but I could see..
- [css] - perhaps someone makes a tool to help edit and manage CSS files; in my experience, it can be a pain, and I'd like to see something better than Visual Studio for that purpose
- [algorithm] - without really looking into it, I found SciCode; no idea what they do or if they'd be a good fit, but they see to be all about algorithms
- [tdd] - how about a company that makes unit and mock testing tools
Keep in mind that we work very hard towards relevance. We won't sell ads, for example, to a Java component company that wants to buy c#-tagged questions. As such, you're not going to see crap like:
Do you like tdd? Find tdd for a great price on NexTag!
Unless, of course, they throw in a yacht (50ft min). You can't blame anyone for selling out for a yacht. And if you do... it won't matter because we won't be able to hear you from our yacht anyway.