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:
http://cherianajay.hubpages.com/hub/extented-HTML-tags[
^].
However, I would strongly advise against doing it. I would advise to read this discussion and thing thoroughly:
http://stackoverflow.com/questions/8906448/custom-tags-why-not[
^].
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.
—SA