Frank Chimero on why designers should learn to code:

Design decisions are not only affected by the characteristics of the content being designed, but also the qualities of the format. The best way to understand the characteristics of the web is to speak its language.

Good design and good markup provide structure to content. Good markup is a fundamental part of good design: beautiful on the inside, beautiful on the outside. HTML and CSS give another venue to provide structure to content in the native language of the web, and learning these guides decisions by surfacing the affordances of the medium. Design decisions are affected by both the content and the format, like how a sculptor would make different decisions if she were working with clay rather than marble.

Spot on. The whole post is worth a read, and Frank gives some good suggestions for resources to help designers get started on coding.