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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.

Figure 6-8

Figure 6-8. Setting the border color for images

Border colors, and borders in general, are all discussed in muchgreater detail in the Chapter 7, "Boxes and Borders".

6.1.1.3. Inheriting color

By this time, you may have guessed thatcolor is inherited, and you're right.

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.



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8.2.2.3. More than one auto

Now let us consider the cases where two of these three properties are set to auto. If both the margins are set to auto, then they are set to equal lengths, thus centering the element within its parent, as you can see from Figure 8-14:

UL {margin-bottom: 10px;}
LI {margin-top: 10px; margin-bottom: 20px;}
H1 {margin-top: 28px;}

Therefore, the last item in the list has a bottom margin of 20 pixels, the bottom margin of the UL is 10 pixels, and the top margin of a succeeding H1 is 28 pixels. Given all this, once the margins have been collapsed (or, if you prefer, overlapped), the distance between the end of the LI and the beginning of the H1 is 28 pixels, as shown in Figure 7-17.

7.7.3. List Item Positions

There is one other thing you can do toinfluence the appearance of list items under CSS1, and that'schange the position of the bullet itself, in relation to the contentof the list item. This is accomplished withlist-style-position.

There is one other important aspect of vertical formatting, which is the collapsing of adjacent margins. This comes into play when an element with declared margins immediately follows another such element in the document's layout. This was discussed in the previous chapter, using this example:

LI {margin-top: 10px; margin-bottom: 20px;}

Padding and borders, where they exist, are never collapsed. If neither is declared, then both will default to 0