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 was unforeseen by the creators of the original XML specification: tag name conflicts.
As we saw in the previous chapter, two popular XML applications are XHTML (that is, HTML 4 as written in XML) and MathML, which lets you display equations. XHTML is useful because it lets you handle all the standard HTML 4 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, both applications define <var> and <select> elements).
The solution ...
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