JS: browser AND platform detection COMBINED

I'm a project manager on a non-profit development team.  Unfortunately, my 
volunteer JS coder can't figure out this problem.

I want to detect for a browser AND platform.  Based on the answer, a CSS 
document will be linked.  Logically speaking, this should be five lines of 
code.

if browser == IE && platform == PC, then ...  get CSS for IE/PC
else if browser == IE && platform == MAC, then ...  get CSS for IE/MAC
else if browser == NN && platform == PC, then ...  get CSS for NN/PC
else if browser == NN && platform == MAC, then ...  get CSS for NN/MAC
else get CSS for OTHER

I've seen free code on irt.org for simple browser detection, but now I need 
to add in the platform detection.  

Thanks for your help.  If you have a solution, please email me directly in 
addition to the list. :)
++Dan

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