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computer monitor, or even a television, and you got far enough into the projection tube, you would discover that there are three "guns." (Remember, however, that actually looking for these guns will pretty much void your monitor's warranty.) These guns shoot out beams of light in varying levels of light and dark, in one of the three RGB colors, at each point on the screen. The brightnesses of each of these beams combine at each point to form all of the colors you see on your screen. Each point, by the way, is known as a pixel, which is a term to which we'll return later in the chapter.
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medium ). Figure 8-10 provides ahandy illustration for remembering which parts of the box can take avalue of auto, and which cannot.

Figure 8-10

Figure 8-10. Horizontal properties that can be set to auto

width must be set to eitherauto or a non-negative value of some type. CSSalso allows browsers to set a minimum value forwidth, below which a block-level element'swidth cannot drop. The value of this minimum can0% or a number divisible by 20; for example, rgb(40%,100%,80%) or rgb(60%,0%,0%). If you use RGB values on the 0-255 scale, then values should be either 0 or divisible by 51, as in rgb(0,204,153) or rgb(255,0,102).

With hex pairs, the appropriate values are 00, 33, 66, 99, CC, and FF. Any hex-pair

XML is totally extensible

By not predefining any tags in the XML Recommendation, the W3C allowed developers full control over customizing their data as they see fit. This makes XML very attractive to encoding data that already exists in legacy databases (by using database metadata, and other schema information). This extensibility of XML makes it such a great fit when trying to get different systems to work with each other.

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.

unless you set different values for the position, background imagesalways start tiling from the top left corner of the containingelement. Length values

Finally, we turn to length valuesfor positioning. When you supply lengths for the position of thebackground image, they are interpreted as offsets from the top leftas possible, a right-floating element as far to the right as possible. A higher position is preferred to one that is further to the right or left.

Again, this rule is subject to restrictions introduced in the preceding rules. There are similar caveats here as in Rule 8, although they are not quite so fuzzy. As you can see from Figure 8-38, it is pretty easy to tell when an element has gone as far as possible to the right or left.

Figure 8-38

Figure 8-38. Get as far to the left (or right) as possible