why jquery's ui.css files's font-sizes are bigger than normal?

Question

Im looking at jqueries tabs, datepicker etc and see that font-sizes of the classes added to elements are like 1.1em, 1em which seems big in the page. Why did they do this like that? What is the purpose? http://jquery-ui.googlecode.com/svn/tags/latest/themes/base/jquery.ui.theme.css

Answer 1

em is a relativ size and depends on the next fixed font-size up the dom tree. when there is no fixed font-size it defaults to 16px.

so when you change your own css like this

body {
    font-size: 12px;
}

1em means 12px, 2em means 24px and so on.

Answer 2

I usually set the class ui-widget in my css like this

.ui-widget {
    font-size:80%;
}