Semantic (X)HTML
The (X)HTML in your Web pages should be semantic. In other words, you should use XHTML or HTML tags to describe what an element is, not what it looks like. For example, in the p element
<p>This is a paragraph.</p>
the <p> and </p> tags mean, respectively, “a paragraph starts here” and “a paragraph ends here.”
If you think of the <p> and </p> tags simply as a way to start a new line, you might be tempted to use them to create a list. You should, however, use the <ul> and <li> tags to create an unordered list:
<ul> <li>This is a list item.</li> <li>This is a list item.</li> </ul>
To summarize, think of (X)HTML tags as ways to describe the structural units—headers, paragraphs, images, and so on—making up your Web page. Don't think of tags as ways to describe appearance.