Sometimes you can know, sometimes you cannot.
If the HTML is generated on the server-side you may not know which programming language generated it. These languages include: PHP, C++, and many other languages. On the server-side, for as many ways as you can think of to guess which language it is, there are just as many ways to for the technology to hide itself.
Suppose you are a web administrator that wants to hide the server-side technology. Pick one of the techniques listed in another question for attempting to identify the language. For example, the *.php extension for a file. Now, configure your web server to execute C code from a file with a *.php extension. Your users will have no way to view the source (since both languages are equally capable of producing the same output, by Turing completeness), but they will be misled into thinking you are running PHP.
Why would someone want to obfuscate the server-side choice of technology? Because CGI languages have various vulnerabilities that are easier to target if the end-users know which of those languages you are using. Misleading the users about which server-side technologies you are using is a very reasonable security measure.
I think that it's fundamental for security testers to gather information about how a web application works and what language it's written in.I think that, if even a security tester can't figure out what language the site is built in, that makes it more secure because then no one will know which exploits to try. (Yes, there are occasionally valid use cases for security through obscurity.)