Tài liệu Creating Class Styles

The CSS class works in a very similar way to the elements’
style definition. You define the CSS class either in the style region
within your HEAD element or in the CSS style document. The
following
is an example of a CSS class style.
.mainTitleStyle {
font-family: Cambria, Cochin, Georgia, Times,
“Times New Roman”, serif;
font-size: 30px;
font-weight: bolder;
color: #008000;
}
As you can see, the main structure for defining the class style
is the same as an element. The difference is that the class is identified
by a leading period and the class name is all one word. You
cannot use spaces in your class name.
After you have created your style you can apply it to any element
in your web page. The element attribute class is used to
associate the element with the CSS class. Here is an example.
<p class=“mainTitleStyle”>Lorem Ipsum Header</p>
<p>In vestibulum, ipsum consectetur cursus porttitor, mi
tellus euismod purus, ac egestas nisl risus ac risus. </p>
<p class=“mainTitleStyle”>Lorem ipsum dolor sit amet</p>
<p>Aliquam sollicitudin elementum est, commodo gravida
lorem imperdiet ac. </p>
<p class=“mainTitleStyle”>consectetur adipiscing elit</p>
<p >Lorem ipsum dolor sit amet</p>