Re:Compatibility with existing browsers (Score 1)

XML is essentially a simplified version of SGML. One of the simplifications is that every element which is not empty must have an explicit end tag. In SGML one could use the - and o notation to indicate whether the start and end tags, respectively, are required or optional. Thus, for example: is the SGML definition of the paragraph element in HTML 4.0, with the O specifying that the end tag is optional. I don't think the move to XHTML 1.0 will have any significant effect on existing browsers, as it is entirely backward compatible with HTML 4.0.