- A question requires at least one tag to ask.
- A new tag can only be created by users with 300+ rep
- Using a placeholder tag will get you in trouble or, in the worst case, get your question closed
- There is no entry in the FAQ about this. The validation messages don't help either.
Stack Overflow has nearly 35028 tags.
New users are supposed to find their required tag in the list.
In the worst case, say if a new user asks about HTML6 which doesn't have a tag yet, then he can tag with HTML, and:
You should be able to find a "container" tag that is at least close to being relevant. If you can't, that's actually a huge clue that your question is off topic.
The more established the site, the more likely it is that the absence of a tag is telling you something. There's no Frying tag on Stack Overflow, no Magical Items tag on Travel, no Windows-8 tag on SFF - because those questions don't belong there. But the first few Windows 8 questions on Stack Overflow were just tagged Windows. The first few C++ 0x questions were just tagged C++. And so on.
Your actions then are:
Just create the question for the closest tag you can find, and add a note on top of your answer. If you were right about the tag not being available, a high-rep SO user will create your tag in < 60 seconds.