I have been asked this a few times, so I decided to write up a little article on it.
The problem is that when you are using master pages in asp.net, the <HEAD> is usually inside the master page template. So if you are on a page that needs a 1 off change or addition to the style of the rest of the site, you are unable to create a <STYLE> element in your content page.
Well, you CAN create one, but then VS.Net won’t show you the designer for your page b/c it keeps asking you to clean up the HTML problems on your page.
The solution I have used is to create a 2nd content place holder in the master page head. But, I might as well not duplicate effort here, as Rick Strahl has already written the article I am about to write (and apparently, even someone else beat him to the punch).
This works very well for this type of situation.