Another reason to test your work across as many different systems as possible. Some browsers and gateways are very forgiving about these errors, and you may not notice the error if you test on only one system. If you leave off the semicolon, your page may work on some devices but fail inexplicably on others. Use images sparingly. WAP is not the Web.
Screen sizes are small, and download times are long.
Use an image only where a few words of text isn't enough. And don't use images as bullet points or other decorations. Not only does this slow down page display on most devices, but you cannot rely on the image appearing where you want it to on some browsers. Don't miss our exclusive O'Reilly interview with Martin Frost . Keep the files small.