XML Namespaces
There's considerable freedom in XML because you can define your own tags. However, as more XML applications came to be developed, a problem arose that had been unforeseen by the creators of the original XML specification: tag name conflicts.
As we saw in Chapter 1, two popular XML applications are XHTML—that is, HTML 4.0 as written in XML—and MathML, which lets you display equations. XHTML is useful because it lets you handle all the standard HTML 4.0 tags; if you need to display equations, MathML can be essential. So what if you want to use MathML inside an XHTML Web page? That's a problem because the tags defined in XHTML and MathML overlap (specifically, each application defines a <var> and a <select> element).
The solution ...
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