I think a lot of this depends on what you mean by "backend". Personally, I need a designer who can write the frontend. That means HTML/CSS, but it also means JS (because what is a web app without JS these days). More importantly, it means being able to work directly on the app without hand-holding - for example, knowing where the CSS goes in a Rails project and being able to work on it directly.

I wouldn't expect my designer to write a sorting algorithm, or a web crawler, or anything that's more code-y, but HTML/CSS/JS and the frameworks which hold them are part of the toolset.