Drupal sites often need to change and add new functionality after launch, which can lead to design failures if the original design was too specific. The document provides tips for designing generically in Drupal to avoid this issue, such as styling all HTML elements, creating generic page, block, and content list designs, and accounting for common Drupal elements like expanding/collapsing regions, menu trees, tabs, and pagers. Designing at a more abstract level following these principles helps ensure a Drupal site's design remains coherent as new features are added.