Comment Re:How exactly do you measure this? (Score 2) 585
User Agent strings aren't the only way of identifying browsers. Generally these days, you do UA strings and object detection. Basically the latter is running JavaScript with a whole bunch of if statements to see if certain objects are defined. document.all is an IE only thing, and window.performance only exists in IE9 for instance. window.opera only exists in Opera (duh).
With WebKit browsers (Chrome, Safari), you can detect to see if they have Canvas and WebGL support. With IE, you can even use conditional comments.
If you have a UA string claiming to be Firefox 2 but it responds to document.getElementsByClassName, you know something is lying to you.
To see how this sort of thing works, take a peek at http://www.quirksmode.org/js/detect.html