For the best user performance, you should design different websites for each mobile device class, says Jakob Nilsen. The smaller the screen, the fewer features, and the more scaled back your design. The very best option is to go beyond browsing and offer a specialized downloadable mobile application for your most devoted users. In practice, however, only the biggest and richest sites can afford all this extra work on top of their desktop-optimized website.

Moderately rich sites should build two mobile designs: one for low-end cellphones and another for smartphones and big-screen phones. This strategy is especially good if you're targeting a broad consumer audience with many feature-phone users. The small-phone experience is so different that it needs a dedicated and deeply scaled-back design, whereas the bigger phones benefit from a design that's mobile-friendly but not bare-bones. Feature-phone browsing is essentially a linear experience, whereas smartphone and full-screen browsing provide more of a GUI experience ? albeit through a limited viewport.

For many sites, however, the only realistic option is to supplement the main site with a single mobile site, recognizing that it will serve feature phones poorly. This strategy often makes sense. After all, most low-end mobile users suffer such misery when they attempt to visit websites that they do so only for the most compelling tasks, and thus might not use your site anyway.

Get the full story at Jakob Nielsen's blog