Web users waiting for pages to load find their days filled with innumerable small delays-long enough to break the rhythm, but too short to do other work. These moments represent prime real estate to Internet marketeers, who see the gaps as a perfect opportunity to squeeze in a commercial message. That's the logic behind "webmercials"-a new form of advertising that should begin appearing in the coming months. KMGI.com, a New York ad agency, is creating eye-popping animations that appear on the screen during gaps in an online session. The webmercials use vector graphics, which describe an image with succinct mathematics rather than specifying each pixel. Thus tiny files-typically 25 kilobytes-can deliver audiovisual spots lasting 5 to 7 seconds.