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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) 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.

9.3. Absolute Positioning

Since most of the examples and figures in the chapter (besides the previous section) have been examples of absolute positioning, we're already halfway to an understanding of how it works. Most of what remain are the details of what happens when absolute positioning is invoked.

When an element is positioned absolutely, it is completely removed from the document flow. It is then positioned with respect to its

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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if the user agent doesn't have a predefined rule such as OL {list-style-type: decimal;}, it may prohibit ordered bullets from being applied to unordered lists, and vice versa. You can't count on this, of course, so be careful.

If you wish to suppress the display of bullets altogether, then none is the value you seek. none will cause the user agent to refrain from putting anything where the bullet would ordinarily be, although it

On the other hand, we might want H1 elements tohave uneven padding and H2 elements to have regular padding, as shownin Figure 7-57:

H1 {padding: 10px 0.25em 3ex 3cm;} /* uneven padding */H2 {padding: 0.5em 2em;} /* values replicate to the bottom and left sides */
Figure 7-57

Figure 7-57. Uneven padding

It's a little tough to see the padding, though, so let'sadd a background color, as shown in Figure 7-58:

SPAN.dropcap {font-size: 300%; font-weight: bold; float: left;width: 0.75em;}<P><SPAN CLASS="dropcap">T</SPAN>his is a paragraph with...</P>

Since this is very similar to the fictional tag sequence used todescribe the behavior of :first-letter anyway, itworks fairly well. It's less elegant, granted, but it doeswork. We use a width of 0.75embecause most letters are not as wide as they are tall, but of courseyou may use other values; experiment to see what you like best.

The block-level <CENTER> tag is still a commonly-usedalternative (includes its own line breaks).  Use <BLOCKQUOTE>for both-margin indents, <UL> with no list items forleft-indents (see below). 
 You can terminate list items with a </LI> tag but it'snot required. Can you nest sub-lists within lists? Soitanly! Levels aredifferentiated by indent and bullet or number styleOrdered lists let you specify TYPE and an ordinal VALUE for a listbolder) will inherit the value of 100 and then evaluate to the next-heaviest face, which is the Bold face and which has a numerical weight of 700. Figure 5-11 shows us the visual result of all this.

Figure 5-11

Figure 5-11. Greater weight will usually confer visual boldness

Let's take this all one step further, and add two more rules, plus some markup, to illustrate how all this works (see Figure 5-12 for the results):