HTML – Content Structure
CSS – Presentation
In web design/development they all inter-relate. But you don’t have to know one necessarily to do the other. However, having understanding of each of the ones you’re not an expert on is necessary in this day and age.
For example, if you are developer and you’re trying to present an idea to your boss. Instead of wasting time in the meetings explaining you might just dive in make a demo and present the solution. This might require little or a lot of HTML/CSS/JS. If you work for larger organization this might be split up between many members of your team.