Using CSS for a fade-in effect on page load

Question

Can a text paragraph fade in on page load using CSS transitions?

I adore the way a fade-in effect looked on a site I once saw and would love to achieve a similar result using only CSS. The site is no longer available, but you can still view an archived version on the Wayback Machine.

Illustration

This markup is present:

<div id="test">
    <p>​This is a test</p>
</div>​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​

With the following CSS rule:

#test p {
    opacity: 0;
    margin-top: 25px;
    font-size: 21px;
    text-align: center;
    -webkit-transition: opacity 2s ease-in;
    -moz-transition: opacity 2s ease-in;
    -o-transition: opacity 2s ease-in;
    -ms-transition: opacity 2s ease-in;
    transition: opacity 2s ease-in;
}​

How can the transition be triggered on load?