I have never understood why some people say making custom css for each browser is a bad thing. To keep my page size down and download times fast it makes perfect sense to me to make a custom css for the major browsers (especially IE in its many different forms), and a general catch all css for everything else.
If you want to send out a bloated, huge, Swiss army knife of the css world, for all situations then go right ahead I'm not going to stop you.
Fast detection of the browser is important when doing this. Loading a JavaScript file to detect the browser seems slow. So I would prefer to use php to detect the browser, and send out the specified css. Or at least a general browser specific css then use the JavaScript to load a more detailed version of the css.
But I've read article after article about why this is a bad thing. The main reason behind each of these articles is because the user agent can be faked. Or there using Firefox but the server thinks they're using IE7 so it sends out the wrong css file.
As a developer/designer of web apps why is this my problem? If you want to use Firefox, but tell my server your using safari or IE*, and get a crappy looking page, why is it my problem?
And don't throw that whole if the user can't see your site right they'll never come back, or some kind of similar argument at me. a normal user isn't going to be doing this. its only going to be the people who know how to do this, and will know whats wrong when my site looks crappy.
This is similar to looking at my site on a old Apple II (I have no clue how), and yelling at me because everything looks green.
So is there a good reason, not a personal preference, why I shouldn't use php to detect the browser and send out customized css files?
I do this mostly for the different versions of IE. It just seems like for some sites, adding the if IE6 and if IE7 parts just double or triple the size of the css file.
As long as the only thing you're doing is changing style sheets, there is no valid reason as far as I can tell. If you're attempting to deliver custom security measures by browser, then you'll have issues.