Quote - you do realize that there are codes to determine what browser the guest is using, right?  the code can direct the user to the correct CSS for their browser...i used TopStyle to create my CSS...been a long time since i designed a site, but would imagine there's tons of stuff out there to help with the browser specific differences and how to deal with them.

User agent strings lie and require consistant updating since Chrome and Firefox are on a six week release timeline.  It'd be a bad decision to do that.

Instead, use JavaScript to identify featuresn, and use the results of the true / false tests to determine what events to attach and / or how to display various things.  

Use of media queries correctly would allow you to design for various display breakpoints.  It seems that the current theme in use is almost entirely percentage based measurements, which is why the layout breaks when you are inbetween commonly used sizes.

There is a difference between a Responsive layout and a fully fluid one, and some designs (like this site) do not work well in a fully fluid state.