Custom XML tags??? Custom as in "< name >", "< footer >", "< my_footer >" or any one of thousands or millions of imaginable custom tag names that extensible markup languages were created for?
You use XML to create a custom markup language. The level of complexity of your new markup language will depend on what you are using it for. The greater the complexity of the structure you want describe, the greater the complexity of your document type. The number of tags can get very large, and the rules for combining those quite complex.
If your spend a lot of time and money to develop a markup language in XML and want to give it away, I will commend your generosity. But if you get a patent for it and want to charge people for using it, and you can find people to pay you for using, I will also commend you for being a good business man.
Since an XML markup language is not compiled, anyone can read it. So it's very easy to copy. The merit of this case will be based on the validity of i4i's patent.