It means that a 300px wide image won't fit inside a 300px wide box any more. You need to decide whether your sizing for each element depends on your layout or

It means that a 300px wide image won't fit inside a 300px wide box any more. You need to decide whether your sizing for each element depends on your layout or your content, and choose accordingly. If one way is more useful to you nearly all of the time, then use it that way. However, also be aware that the star rule is the least performant rule in CSS (http://stackoverflow.com/questions/1714096/why-is-the-css-star-selector-considered-harmful http://stackoverflow.com/questions/1714096/why-is-the-css-st...).