After years of developing websites and online software, one of the most frustrating issues I have had to deal is cross-browser compatibility. From a CSS perspective, this typically meant either 1) Writing browser-specific hacks (mostly IE6) or 2) developing multiple style sheets for each major browser. Although this is still a reality in some cases, one of the most important tools I have found is the CSS Reset stylesheet.
Simply put, this stylesheet strips out all of the "Built-in" styles that exist in major browsers and allows the developer to start coding CSS on a level playing field. I have found that starting your stylesheet with this technique will help eliminate many problems that are typically encountered down the road.
Many other examples of this technique exist on sites like Yahoo and Six Revisions
