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10.4. Generated Content

Generated content is a new way of adding things to existing content without having to alter the content itself. It's done by using the pseudo-elements :before and :after and the property content. Here's a basic example of how it works:

P:before, P:after {content: "\""; color: gray;}
<P>This is a quote.</P>

The browser will display what's shown in Figure 10-15.

Figure 10-15

Figure 10-15. Adding generated content

Note that the double-quote mark was escaped out -- that is, preceded by a backslash. This is necessary, since text values for content must be enclosed in double quotes. You could also place images before (or after) content, using something like P:before {content: url(para.gif);} to put a paragraph symbol at the beginning of each paragraph. You can even string multiple values together like this (shown in Figure 10-16):

P:before {content: url(para.gif) " -- ";}
Figure 10-16

Figure 10-16. Adding an image and text before a paragraph

This would cause each paragraph to be started with a paragraph symbol, a blank space, two dashes, and then another blank space. Note that all of this is considered part of the paragraph and is inlined within it. The spaces appear before and after the double dash because they're included in the string value. If these spaces were omitted, then space would not appear to either side of the dashes.

Let's say, though, that you want to do some real quoting, using real quotation marks -- you know, the curly double quotes that are so hard to specify in HTML and which often don't show up even if you do try to specify them. CSS2 has ways to handle this.

content has some other values you can use:

So if you wanted your quotations to begin and end with quotation marks, instead of typing in a literal quotation mark, you could let the browser insert "smart quotes" for you.

BLOCKQUOTE:before {content: open-quote;}
BLOCKQUOTE:after {content: close-quote;}

10.4.1. Automatic Numbering

In the same vein, CSS2 also includes properties for automatic numbering. First, you can specify a counter as a value of content. This can be a bit tricky, and it would take too long to run through all the possibilities, but here's an example. Say you wanted the chapters and sections of a document

<BODY BACKGROUND="bg23.gif">

This will cause a user agent to load the filebg23.gif and then "tile" it in thedocument background, repeating it in both the horizontal and verticaldirections to fill up the entire background of the document, as shownin Figure 6-22.

Figure 6-22

Figure 6-22. Applying a background image in HTML

This effect can be duplicated in CSS, but CSS contains a great dealmore than simple tiling of background images. We'll start with numbered 1, 1.1, 1.2, 1.3, and so on. In addition, you're using H1 for your chapters and H2 for your sections. Here are the declarations you would use:

H1:before {
content: "Chapter " counter(chapter) ". ";
counter-increment: chapter;   /* Add 1 to chapter */
counter-reset: section;       /* Set section to 0 */
}
H2:before {
content: counter(chapter) "." counter(section) " ";
counter-increment: section;
}

As we can see from Figure 10-17, the user agent will add the word "Chapter" and a number at the beginning of H1 text. This number is automatically incremented with each H1, due to the declaration counter-increment: chapter;. It also sets the section counter back to zero through counter-reset: section;. Then, for each section heading (H2), the browser uses the chapter number, followed by a period (.) followed by the current section number, which is also automatically incremented.

Figure 10-17

Figure 10-17. Adding counters to elements

You don't have to increment by one every time, either. You can define any integer as the increment value, including zero and negative numbers. If you want each section to have an even number, as we see in Figure 10-18, then you can declare the following:

H2:before {
content: "Section " counter(section) ". ";
counter-increment: section 2; /* Add 2 to chapter */
}
Figure 10-18

Figure 10-18. Changing a counter's incremental value

You can also keep an element from incrementing a counter by setting its display to none. Of course, that will cause the element to disappear altogether.

10.4.2. Markers

You can do even more by using the value marker for the property display, which enables you to define your own marker styles for any element at all. You're already familiar with markers, as it happens -- the bullets and numbers at the beginning of list items are markers.

Let's say we want to recreate the way unordered lists behave. For the purposes of this example, we'll use the image disc.gif to stand in for the normal bullets. Using marker properties, we would declare:

LI:before {display: marker;
content: url(disc.gif);
marker-offset: 1em;
}

This will insert the disc image before each list item, and set it to be offset from the left edge of the LI content by 1em, as shown in Figure 10-19.

Figure 10-19

Figure 10-19. Styling list markers

Marker properties are not restricted to list items, however. Let's say that, throughout a document, there are a few paragraphs with a class of aside. We wish to call attention to these paragraphs by inserting a small note to the side of each one. Here's one way to do it:

BODY {counter-reset: aside-ctr;}
P {margin-left: 10em;}
P.aside:before {display: marker;
content: "Aside " counter(aside-ctr) " --";
counter-increment: aside-ctr;
text-align: right;
marker-offset: 1em;
width: 9.5em;
}

The effect will be something like that seen in Figure 10-20.

Figure 10-20

Figure 10-20. Automatically numbered asides

This is yet another aspect of CSS2 that, once it's been properly implemented, will allow authors to do quite a bit with their documents.



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been possible to float images by declaring, for instance, <IMG SRC="b5.gif" align="right">. This causes an image to float to the right, and allows other content (text or other images) to "flow around" the image. In the past, this was only possible with images and, in some browsers, tables. CSS, on the other hand, allows any element to float, from images to paragraphs to lists. In CSS, this behavior is accomplished using the property float.

XML supports shareable structure (using DTDs)

Since the structure of the XML document can be specified in DTDs they provide a simple way to make it easier to exchange XML documents that conform to a DTD. For example, if two software systems need to exchange information, then if both of the systems conform to one DTD, the two systems can process information from each other. DTDs are not as powerful as some kind of schema architecture for XML, they don't support typing, subclassing, or instantiation mechanisms that a schema architecture must have.

DTDs are a simple way to make sure that 2 or more XML documents are of the same "type". Its a very limited approach to making "typed" XML documents shareable across systems. In the future some kind of schema system will be proposed by the W3C that should allow typing, instantiation and inheritance of information (in XML).

XML enables interoperability

you have to be to shrink text in lists like that?" True, it's easy to spot this with lists. However, think about how most of your pages are structured (with nested tables) and then consider this rule:

BODY {font-size: 12pt;}
TD {font-size: 80%;}

All it takes is three levels of nesting in your tables, and you end up with 6-point text (12 x 0.8 x 0.8 x 0.8 = 6.144). Many complicated pages have at least three levels of nesting, collapse.

fixed

The element's box is positioned as though it were set to absolute, but its containing block is the viewport itself. In screen media such as web browsers, the element will not move within the browser window when the document is scrolled. This allows for frame-style layouts, for example. In paged media such as right margin of 20 pixels, a bottom margin of 15 pixels, and a leftmargin of 5 pixels, here's all we need:

H1 {margin: 10px 20px 15px 5px; background-color: silver;}

As Figure 7-8 reveals, we have what we wanted. Theorder of the values is obviously important, and follows this pattern:

margin: top right bottom left
Figure 7-8

Figure 7-8. Uneven margins

A good way to remember this pattern is to keep in mind that the four