I don't think you can do this in HTML, which is a markup language, not a programming language. I think what you want to do is write the page with your programming language of choice, and define the variable in that language. For example, here's how I might do it in Python:
person = "John Doe"
print("<h3>", person, "</h3>\n"
"<p> your name is", person, "</p>")