You might want to look at HTTP::BrowserDetect.
And here is why you should: You will need to check for the string "Mozilla", as this is how they (IE and NS) show up as , if found determine if its an IE or a thing that calls itself "gecko". There are plenty of User_agents out there and I am sure a google search will quickly turn up a list of about 50 different strings that those different browsers present to identify themselves, but you might get stuck as the MSIE ADMIN kit allows for customisation which affects that string in your severlog as well, recently I've seen one MCK, which stood for McKinsey.

Have a nice day
All decision is left to your taste
Update :
The W3C is your friend regarding all things and links about Document Object Model (DOM).