A test card is (according to Wikipedia):

A test card, also known as a test pattern in North America, is a television test signal, typically broadcast at times when the transmitter is active, but no program is being broadcast (often at startup and closedown). Originally, all test cards were actually physical cards at which a television camera was pointed, and such cards are still often used for calibration, alignment, and matching of cameras and camcorders. Test patterns used for calibrating or troubleshooting the downstream signal path are nowadays generated by test signal generators, which do not depend on the correct configuration of (and presence of) a camera. (...) Most include a set of calibrated color bars which will produce a characteristic pattern of "dot landings" on a vectorscope, allowing chroma and tint to be precisely adjusted between generations of videotape or network feeds

Some examples:

Why do I blog this? first because I like their odd design; second because it's an artifact that belongs to the past. What's the equivalent of the test pattern for the Web?