A "tag" probably means HTML custom tags (as in XML, all tags are "custom").
In principle, such thing is possible. Here is the short explanation of the technique and usage:
However, I would strongly advise against doing it. I would advise to read this discussion and thing thoroughly:
Read some more on related topics: http://bit.ly/WQ4B8W
For all practical purposes, I would rather believe that you can implement whatever you want without custom tags. But first, you would need to explain what exactly do you want to achieve and why. It's a good idea to start every your question with the explanation of your ultimate goal. This can bring you a good advice even if your main question is based on some preoccupation with a wrong idea.