The Web enables broad dissemination of information and services; however, the ways in which sites are designed can either facilitate or impede users' benefit from these resources. We present a longitudinal study of web site design from 2000 to 2003. We analyze over 150 quantitative measures of interface aspects (e.g., amount of text on pages, numbers and types of links, consistency, accessibility, etc.) for 22,000 pages and over 1,500 sites that received ratings from Internet professionals. We examine characteristics of highly rated sites and provide three perspectives on the evolution of web site design patterns: (1) descriptions of design patterns during each time period; (2) changes in design patterns across the three time periods; and (3) comparisons of design patterns to those that are recommended in the relevant literature (i.e., texts by recognized experts and user studies). We illustrate how design practices conform to or deviate from recommended practices and the consequent implications. We show that the most glaring deficiency of web sites, even for sites that are highly rated, is their inadequate accessibility, in particular for browser scripts, tables, and form elements.