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Index: U

XML documents are also naturally committed to a database (relational or object) or any other kind of XML document store. There are commercial products available which allow you to save XML documents to an XML storage layer (which is not a database per se), like Datachannel's XStore and ODI's eXcelon. These XML store solutions are quite expensive ($10,000 to $20,000 range).

XML documents are also quite naturally retrieved from a persistence layer (databases, file systems, XML stores). This lends XML to be used in real world applications where the information being used by different parts of a system is the most important thing.

XML is platform independent, textual information

Information in an XML document is stored in plain-text. This might seem like a restriction if were thinking of embedding binary information in an XML document. There are several advantages to keeping things plain text. First, it is easy to write parsers and all other XML enabling technology on different platforms. Second, it makes everything very interoperable by staying with the lowest common denominator approach. This is the whole reason the web is so successful despite all its flaws. By accepting and sending information in plain text format, programs running on disparate platforms can communicate with each other. This also makes it easy to integrate new programs on top of older ones (without rewriting the old programs), by simply making the interface between the new and old program use XML.

For example, if you have an address book document stored in an XML file, created on a Mac, that you would like to share with someone who has a PC, you can simply email them the plain text address book XML document. This cant be done with binary encoded information which is totally platform (and program) dependent.

U element: 4.1.6. Text Decoration
UA (see user agent)
underlining: 4.1.6. Text Decoration
changing color of: 4.1.6.1. Weird decorations
removing from hyperlinks: 4.1.6. Text Decoration
turned off by browsers: 4.1.6.1. Weird decorations
Uniform Resource Identifier (URI): 3.5. CSS2 Units
Uniform Resource Locators (see URLs)
units: 11.1.3. Case 3: Putting a Magazine Article Online
(see also length units; CSS2 units)
for aural style sheets: 3.5. CSS2 Units
avoiding mixing: 11.1.3. Case 3: Putting a Magazine Article Online
color: 3.6. Summary
universal selector: 10.2.1.1. Universal selector
unordered lists: 7.7.1. Types of Lists
unvisited anchors: 2.4.1. Pseudo-Class Selectors
uppercase text: 4.1.5. Text Transformation
upright text: 5.4.1. Fonts with Style
URI (Uniform Resource Identifier): 3.5. CSS2 Units
URLs (Uniform Resource Locators): 3.4. URLs
HREF attribute and: 1.4.1.1. LINK attributes
referring to in style sheets: 3.4. URLs
specifying for images: 6.2.1. Background Images
user agent (UA): 2.2.2. Grouping Declarations
2.2.2. Grouping Declarations
(see also browsers)
users, selecting alternate style sheets: 1.4.1.2. Alternate style sheets


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Figure 7-7

Figure 7-7. Comparative paragraphs

(Again, the background color helps show the content area, and thedashed lines are for illustrative purposes only.) As Figure 7-7 demonstrates, 10 pixels of space are added toeach side of the content area. This is somewhat similar to using theHSPACE and VSPACE attributes inHTML. In fact, you can use margin to set extraspace around an image. Let's say you want 1 em of spacesurrounding all images:

That's all it takes.

All of the code that you write (in your Java classes) might be considered the Java application layer. Other layers are the XML Parser layer, the XML source (that supplies the XML data that is necessary), and the persistence engine (where the data is actually stored and retrieved by the source).

Your code (in the Java application layer) has to make use of the DOM or SAX API and the XML parser in order to access the information in XML documents (that come from your source). The source might be responsible for pulling data from different persistence engines (relational or object databases) and even the web (dynamically generated websites that supply only XML data).

In your application layer, you can create many interesting Java applications. The apps can run on the server side or client side or both. They may have graphical user interfaces or they may be web based. When I use the word application or app in this chapter, I don't exclude Java applets; I mean application (or app) in the broad sense of the word, i.e., I mean it to describe a software system written in Java that solves a real-world problem.

3 Main categories

inline element, and the edge of the inline element's background. This can end up on both ends of the inline if we wish:

B {margin: 10px; background: silver;}

As expected, Figure 7-24 shows a little extra space on the right and left sides of the inline element, and no extra space above or below it.

Figure 7-24

Figure 7-24. An inline element with a 10-pixel margin

This all seems simple enough, but when the boldfaced text stretches