Worth 117 words
The alt attribute in (X)HTML is designed to provide alternative text for an image whenever the image can’t be loaded or displayed. (IE shows it as a tooltip.)
“A picture is worth a thousand words.’ That saying of course is just an exaggeration. According to the USDA Forest Service, it’s worth only 117 words. Imagine seeing that as the tooltip for an image on a webpage you’re visiting:
A static image (updated every five minutes) of Mount St. Helens, Washington USA, taken from the Johnston Ridge Observatory. The summit of Mount St. Helens is at an elevation of 2,549 Meters (8,364 feet), at 46.20 N, 122.18 W. The summit stood at 9,677 feet before the May 18, 1980, eruption. The Observatory and VolcanoCam are located at an elevation of approximately 4,500 feet, about five miles from the volcano. You are looking approximately south-southeast across the North Fork Toutle River Valley. The Mount St. Helens VolcanoCam is brought to you by the U. S. Geological Survey Cascades Volcano Observatory, Vancouver, Washington, Gifford Pinchot National Forest, Vancouver, Washington, and Mount St. Helens National Volcanic Monument, Amboy, Washington USA.
(X)HTML also provides a longdesc attribute so you can direct the visitor to a page with more information on that image. Here’s the URL for that image’s longdesc:
http://www.fs.fed.us/gpnf/volcanocams/msh/A%20static%20image%20(updated%20every%20five%20minutes)%20of%20Mount%20St.%20Helens,%20Washington%20USA,%20taken%20from%20the%20Johnston%20Ridge%20Observatory.%20The%20summit%20of%20Mount%20St.%20Helens%20is%20at%20an%20elevation%20of%202,549%20Meters%20(8,364%20feet),%20at%2046.20%20N,%20122.18%20W.%20%20The%20summit%20stood%20at%209,677%20feet%20before%20the%20May%2018,%201980,%20eruption.%20The%20Observatory%20and%20VolcanoCam%20are%20located%20at%20an%20elevation%20of%20approximately%204,500%20feet,%20about%20five%20miles%20from%20the%20volcano.%20You%20are%20looking%20approximately%20south-southeast%20across%20the%20North%20Fork%20Toutle%20River%20Valley.%20The%20Mount%20St.%20Helens%20VolcanoCam%20is%20brought%20to%20you%20by%20the%20U.%20S.%20Geological%20Survey%20Cascades%20Volcano%20Observatory,Vancouver,%20Washington,%20Gifford%20Pinchot%20National%20Forest,%20Vancouver,%20Washington,%20and%20Mount%20St.%20Helens%20National%20Volcanic%20Monument,%20Amboy,%20Washington%20USA.
Nice and succinct.
Thanks to Eric Meyer for the scoop.