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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)

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

<length> | <percentage>

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 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.



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Having done this, we need a style declaration that will recreate theeffects of all the tags we just deleted. This should just about dothe trick:

.sidebar .head {font-size: larger; font-weight: bold;text-decoration: underline; color: white;}

By using the contextual selector .sidebar.head, we ensure only those.heads inside a .sidebar willreceive these styles. Since the entire sidebar is already set to use let's say you want to eliminate the space betweenH1 elements and paragraphs. Here's thesimplest case for doing so:

H1 {margin-bottom: 0;}P {margin-top: 0;}

This is, after all, one correct way to eliminate the space betweensucceeding elements. Navigator 4.x, however, will display theelements with the usual blank line between them, as you can see inFigure 7-26. This is because it's adding thescroll along with the document. The second is that the position ofthe image is determined by the viewing area, not the document size.

Figure 6-55

Figure 6-55. Nailing the background in place

When printed, of course, the two are the same, since the display area(the paper) is the same as the document size, at least for that page.In a web browser, though, the viewing area can change as the userresizes the browser's window. This will cause thebackground's origin image to shift as the windowchanges size. Figure 6-56 depicts several views ofresizes the browser's window. This will cause the background's origin image to shift as the window changes size. Figure 6-56 depicts several views of the same document. So in a certain sense the image isn't fixed in place, but it will remain fixed so long as the viewing area isn't resized.

Figure 6-56

Figure 6-56. Centering still holds, even if the image is "fixed"

There is only one other value for background-attachment, and that's theCSS1 (although that property, and others like it, were introduced inCSS2). You can, however, sneak around this limitation by declaringthe style for a given border using one of the shorthand propertieswe'll discuss later in the chapter.

7.4.1.2. Falling back on solid

There is one interesting thing about CSS that can make life difficultfor authors. According to CSS1, a user agent is allowed to interpret