Here's a designer's take on why this is a great answer by pj - the html/css is the meeting ground between designers and programmers, so without a basic understanding goes a long way.

Think of it this way - html by itself, even without css, is like the skeleton of a body. The programming (backend & javascript) is like the muscles that move stuff around and the css & visual design is the skin, makeup and clothes that define the style. It's a bit simplistic and ignores more advanced interaction design that can help define the skeleton in the first place, but it's a good shorthand for how designers and programmers make stuff, especially in early stages.

So that's a hell yes to learning html/css. Learning web design means more than learning simply the toolkit to create those designs, it's also about developing an eye for what makes an attractive or usable interface, so learning whatever you can there should probably be a hobby that teaches you to appreciate good work. Try cruising the css gallery sites or smashingmagazine for examples.