CSS: The Missing Manual, 4th Edition

Chapter 7. Margins, Padding, and Borders

Every HTML tag is surrounded by a world of properties that affect how the tag appears in a web browser. Some properties—like borders and background colors—are immediately obvious to the naked eye. Others, though, are invisible—like padding and margin. They provide a bit of empty space on one or more sides of a tag. By understanding how these properties work, you can create attractive columns and decorative sidebars and control the space around them (what designers call white space) so your pages look less cluttered, lighter, and more professional.

Taken together, the CSS properties discussed in this chapter make up one of the most important concepts in CSS—the box model.

Understanding the Box Model

You probably think of letters, words, and sentences when you think of a paragraph or headline. You also probably think of a photo, logo, or other picture when you think of the <img> tag. But a web browser treats these (and all other) tags as little boxes. To a browser, any tag is a box with something inside it—text, an image, or even other tags containing other things, as illustrated in Figure 7-1.

Surrounding the content are different properties that make up the box:

• padding is the space between the content and the content’s border. Padding is what separates a photo from the border that frames the photo.

• border is the line that’s drawn around each edge of the box. You can have a border around all four sides, on just a single side, or any combination ...