How do I define a constant of entity in in HTML?

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:


Perhaps I can give you a better answer if you give some more specifics. For example, are you just looking for a shortcut in creating static webpages? Or are you looking to create an interactive, dynamic webpage? (In the latter case, you probably want to embed a piece of Javascript in your HTML.) What programming languages are you familiar with? What is operating system does your webserver run on, and what kind of access do you have to it?

I don't need a "variable". I want a static constant which I declare as some value once. Like a constant in VB.

There is nothing like that in HTML. You can embed Javascript in HTML which is where I would start if I wanted to do something like this client side. It is hard to design it without knowing exactly what you are trying to do.

Generally when a professional wants to write something like this, they will either use a template feature on the server side (i.e. JSPs in Java or PHP templates), or a client side javascript library (e.g. jquery or backbone).

Although that's a perfectly reasonable thing for you to want, HTML has no obligation to give it to you. And to the best of my knowledge, it doesn't. To get done what you want done, you'll need to enlist the help of some programming language. I gave you an example using Python. Maxdancona's JavaScript suggestion makes sense, too.

On a broader note, there's no use in telling us what you need from a language. We can't rewrite the language definition for you. But maybe we can help you accomplish your task. So why don't you tell us what your task is?