As usually, the key is in the navigator.appVersion property. In Internet Explorer 5.5, for example, this property looks like this:

4.0 (compatible; MSIE 5.5; Windows 98)

To extract the string "5.5", we would first get the position of "MSIE":

navigator.appVersion.indexOf("MSIE")

Then add 5 to get to the position of "5.5":

msieIndex = navigator.appVersion.indexOf("MSIE") + 5;

Then extract the three-character-long substring beginning at msieIndex:

navigator.appVersion.substr(msieIndex,3)

Finally, convert the string to a floating number:

parseFloat(navigator.appVersion.substr(msieIndex,3))

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