I’d say if you want to be a web designer then having a working knowledge of HTML would be pretty much be a prerequisite. But not necessarily… an art director I once worked at a web design studio didn’t even know what H T M L meant…

If you aren’t proficient in HTML, you may be wondering why it is necessary to learn HTML when there is so much else you need to know. Knowing HTML can only enhance your productivity and value as a developer. After all, it will be much easier to track down a bug in a Web interface with intricate knowledge of how the page is delivered to the browser. In addition, HTML knowledge (along with CSS) means you can work with an application regardless of the tool used to create it. You are no longer bound to your preferred development tool or the one used to build the site. Also, knowledge of the source code gives you greater control of the end result. You can use your favourite tool to build the application and dig into the source code to tweak it.