Book HomeCascading Style Sheets: The Definitive GuideSearch this book Wednesday 29th of June 2016 09:13:51 AM Remember that absolutely positioned boxes can have backgrounds,margins, borders, and padding; styles can be applied within them,just as with any other element. This can make them very useful forthe creation of sidebars, "stickynotes," and other such effects. One example is the ability toset a "change marker" on any paragraphthat has been edited. This could be done using the following stylesand markup:

SPAN.change {position: absolute; top: 0; left: -5em; width: 4em;

Chapter 8. Visual Formatting

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 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.

pages where the main text is black with a white background, but the navigational hyperlinks along the side of the page are black on a light purple background. There are almost infinite possibilities: defining red text for warnings, using a dark purple to make boldfaced text even more obvious, setting each heading to be a different shade of green, and on and on.

Of course, this means that when you're designing a page, you need to put some thought into it first. That's generally true in any case, but with colors, it's even more so. For example, values for color that it doesn't recognize.If Navigator 4 encounters an unknown word (such asinvalidValue) somehow, through mechanisms knownonly to itself, it actually arrives at and uses a color. Itdoesn't do so randomly, exactly, but the effect is practicallythe same. For example, invalidValue comes out as adark blue, and inherit, which is a valid valueunder CSS2 but not CSS1, will come out as a really awful shade ofyellow-green. This is not correct behavior, but you'll need toremember it as you write your styles.