Book HomeCascading Style Sheets: The Definitive GuideSearch this book Friday 24th of February 2017 01:53:09 PM

Chapter 8. Visual Formatting

Contents:

Basic Boxes
Block-Level Elements
Floated Elements
Inline Elements
Summary

In the previous chapter, we covered a great deal of information on how CSS handles the visual formatting of a document. However, we did this in a mostly practical fashion: lots of explanation about how things work, with only a little lip service paid to the questions of why. In this chapter, we turn to the theoretical side of visual rendering, with only occasional references to the practical.

You may wonder why it's necessary to spend an entire chapter on the theoretical underpinnings of visual rendering in CSS. The main reason is to cover all the bases. I attempted to provide as many and varied examples as possible in the previous chapters, but with a model as open and powerful as that contained within CSS, no book could hope to cover every possible way of combining properties and effects. Every reader of this book will obviously go on to discover new ways of using CSS for their own document effects.

In the course of so doing, you may encounter what seems like strange behavior on the part of user agents. With a thorough grasp of how the visual rendering model works in CSS, you'll be able to determine whether the behavior is a correct (if unexpected) consequence of the rendering engine CSS defines or whether you've stumbled across a bug that needs to be reported. (See Appendix A, "CSS Resources", for details on how to report problems with rendering engines.)

8.1. Basic Boxes

In the rendering of elements, CSS assumes that every element generates one or more rectangular boxes, called element boxes . (Future versions of the specification may allow for nonrectangular boxes, but for now everything is rectangular.) Each element box consists of a content area at its core. This content area is surrounded by optional amounts of the user. Consider the following styles:

DIV#sidebar {position: absolute; top: 0; left: 0; width: 15%; height: 7em;overflow: hidden;}

In such an instance, the clipped content would not be accessible tothe user. This would lead to a situation like that illustrated byFigure 9-10.

Figure 9-10

Figure 9-10. Clipping content with overflow

Finally, there is overflow: padding, borders, and margins. These are considered optional because all could be set to a width of zero, effectively removing them from the element box. An example content area is shown in Figure 8-1, along with the surrounding regions of padding, border, and margins.

Figure 8-1

Figure 8-1. The content area and its surroundings

Each of the margins, borders, and padding can be set using various properties, such as margin-left or border-bottom. The content's background (for example, a color or tiled image) is also applied to the padding, while the margins are always transparent, allowing the background of any parent elements to be visible. In effect, the margins simulate the HSPACE and VSPACE attributes of images, although in a much more sophisticated fashion. Padding cannot be set to a negative value, but margins can. The effects of negative margins are explored later in this chapter.

The borders, on the other hand, have their own rules. Borders are generated using defined styles, such as solid or inset, and their color can be set using the border-color property. If no color is set, then the color of the border is based on the foreground color of the element's content. For example, if the text of a paragraph is white, then any borders around that paragraph will be white, unless a different border color is explicitly declared by the author. If a border style has "gaps" of some type, then the element's background is visible through those gaps; in other words, the border has the same background as the content and padding. Finally, the width of a border can never be negative.

There are differences in how different types of elements are formatted, however. Block-level elements are not treated in the same way that inline elements are, for example, and floated elements introduce a whole new level of complexity. Let's examine each type of element in turn.



Library Navigation Links

Copyright © 2002 O'Reilly & Associates. All rights reserved.

7.5.2. Single-Side Padding

You guessed it: there are properties that let you set thepaddingon a single side of the box, without affecting the other sides.

padding-top, padding-right, padding-bottom, padding-left

Values
default, this will cause all text in the line to be aligned along their baselines, but of course different vertical-align values will have different effects. All of the elements could be top-aligned, for example. We'll return to vertical alignment later in the chapter, but for now will assume that everything is baseline-aligned.

Now the line-height comes into play. Let's assume the following case:

It is important to remember that the side-offset properties define offset from the analogous side (e.g., left defines the offset from the left side) of the containing block, not from the upper-left corner of the containing block. That's why, for example, one way to fill up the lower-right corner of a containing block would use these values:

top: 50%; bottom: 0; left: 50%; right: 0;

In this example, the outer left edge of the positioned element is

P {line-height: 16pt;}

Since the "inherent" line height of 12-point text is 12points, the preceding rule means that there will be an extra 4 pointsof space around each line of text in the paragraph. This extra amountis divided in two, with half going above each line, and the otherhalf below. While there is a distance of 16 points between the lines,this is an indirect result of how the extra space is apportioned.

Now let's look at a slightly more complex case. Take thefollowing example:

Throughout this section, every example has had a repeat value of no-repeat. The reason for this is simple: with only a single background image, it's much easier to see how positioning affects the placement of the first background image. We don't have to prevent the background image from repeating, though:

BODY {background-image: url(bigyinyang.gif);