You should test for every possible, impossible, logical, illogical, expected, and unexpected occurrence. But I want to publish my pages this century. Okay, then make sure that you plan for the somewhat unexpected.
In particular, you want to test in the following three areas:
Predicted ranges and values: Everything that you intentionally put into your Web pages and expect participants to access. This includes all links, images, ALT=wording, forms input sections and expected values, clickable map areas, and so on.
Boundaries: The edges of the envelope of the expected. More problems occur at the edges than in the middle of most computer programs, which is what your HTML-coded Web page actually is. Many programs work perfectly with expected values, correctly ignoring values outside their boundaries, but then fail on those values at the exact boundaries between the expected and the unexpected. For example, making the expected range from one to 100 and putting in an error-checking routine for values less than zero and more than 101 may work fine until someone puts in zero or 101. Make sure you try the boundary values, too!
Outside the boundaries: Everything not included in the two preceding bullets. Of course you cant click on every pixel on every page any more than you can enter every out-of-range value on every form. You can try a few out-of-bounds values and clicks to make sure you havent missed the somewhat obvious. Many mistakes have been found in a program when the programmer sneezed, causing the programmer to click the mouse button with the cursor on a place where no sane user would ever click.
Dont lose sleep over testing absolutely everything. Do approach testing as something that definitely benefits your Web pages if done logically, methodically, and repeatedly during your Web page development and maintenance.
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Revised -- January 16, 1998