You might also want to add an rgb or hex color background for older browsers that don’t support rgba. See http://css-tricks.com/rgba-browser-support/ .

Another small improvement would be to put “background: #2fa9f6 url(images/info.png) no-repeat 14px 14px;” into your “.alert” selector and overwrite the “background-color” and “background-image” for different alert types. That would allow you to change the background-position for all alert types on one spot (the “.alert” selector).

I also notice you define a “.alert p” selector, yet you don’t use a tag anywhere in your HTML. So you could just delete that line.

Except for those little details, it looks pretty nice 🙂