Phone Number Checker
Accepts what looks to be a valid US phone number. Area code and echange should be 3 digits not starting with a zero while the last 4 digits can start with a zero but must all be numeric.
Additional perks include an auto skip feature that tabs to the next cell after the required number of characters are typed. (note: it doesn't do the above checks till submission time) Under expolrer, if you tab to the next cell or shift-tab back to a previous cell, the entire contents of the cell are selected allowing you to type over it. In mozilla, (or more formally, "non explorer 4 or greater running on win32") the cell select doesn't happen. Additionally, when tabbing through already filled out cells (both forward and reverse) the auto skip will skip the user to the next cell because it thinks the cell is already filled out. Explorer doesn't skip if the cell contents are selected, therefore getting around this problem.
View source to see how it all works.