Book HomeCascading Style Sheets: The Definitive GuideSearch this book Thursday 07th of May 2015 05:35:30 AM

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 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 the default value scroll. As you'd expect, this causes the background to scroll along with the rest of the document when viewed in a web browser, and it doesn't necessarily change the origin image's position as the window is resized. If the document width is fixed (perhaps by assigning an explicit width to 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.

Second, all of the inline elements in a given line are aligned according to their values for vertical-align. By 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

P {background-color: gray;}
Figure 6-14

Figure 6-14. Background gray for paragraphs

If you wish the color to extend out a little bit from the text in the element, then you need only add some padding to the mix, with the result shown in Figure 6-15:

P {background-color: gray; padding: 10px;}
Figure 6-15

Figure 6-15. Backgrounds and padding

(Padding will be discussed in detail in Chapter 7, "Boxes and Borders".)

The link could specify the relative URL <A HREF="cadavers.html"> (the default scheme is http://) or the absolute URL <A HREF="http://www.taxidermy.org/~mad_dog/cadavers.html">.  Relative URLs are usually preferable.  If you moved your site to www.weirdos.net, you would have to fix all the absolute URLs; but the relative URLs would work fine.  (They're shorter too.) 

If a URL omits the filename, the browser looks for a file named "index.html" in the specified directory.  If there is no such file, the browser lists the entire directory's contents.  You should name the top-level page of your website "index.html" to prevent this.  Then youthe important part. What matters is the MIME type the server useswhen sending a file. However, since the vast majority of web serversuse a file's extension to decide which MIME type to use whensending the file, it obviously becomes important to have a friendlyserver configuration.

If an external style sheet is sent using the wrong MIME type, thestyle sheet gets mangled into something unusable. If you find thatyou're having this problem, then you'll need to contact