A main reason why your definition isn't used much is because it is highly dependent on the relative brightness of the dot and its background. You can see a dot that has a millionth of the area of an iPad pixel, as long as it radiates the same number of photons on your retina [edit, make that a thousandth or so. Something a million times as bright might kill the cell it lands on before it can send out a signal. http://www.displaymate.com/iPad_ShootOut_1.htm#Backlight_Pow... claims 7W for an ipad backlight; that's over 2W for a million pixels. You wouldn't want to shine a 2W laser into your eye. If you don't believe that, google 2W laser pointer on YouTube]
Landolt C's may be a bit tricky to get right on a web page, but you might try and code a staircase experiment (http://en.wikipedia.org/wiki/Psychophysics#Staircase_procedu...) with illiterate E's.