Paul is right on the mark. The comments will comment out the style for older browser versions that don't support styling or scripts. It's something I just do without thinking anymore.
PHP, a server side language, allows one to include files. However, keep in mind there are other languages available, including, but not limited to, ASP, ASP.net, JSP, and PERL. There is also something known as Server Side Includes (SSI) which, as the name implies, is a simple method for including files. If you're not doing anyother scripting, and your server supports it, I'd recommend this method, as it's very simple.
On the other hand, using PHP or any other server side language, you can better make sure people aren't accessing your included files when they aren't supposed to. Let's say that you have a file, head.php, which you want to include. This file contains the top section of your webpages. But you don't want people accessing this file directly. There are many methods of preventing it. In PHP, one method I've seen used alot is:
all your html
Now, whenever you call the head.php file, you do this:
If someone goes directly to head.php, they won't get anything. This method is preferable over the .inc method, since using a text editor like Notepad will still allow people to use the file, whereas the scripting I provided won't allow them to view the file unless it's included.