Question 21 :
What is well-formed ?
An XML document that is syntactically correct. It does not have any angle brackets that are not part of tags, all tags have an ending tag or are themselves self-ending, and all tags are fully nested. Knowing that a document is well formed makes it possible to process it. However, a well-formed document may not be valid. To determine that, you need a validating parser and a DTD.
Question 22 :
What is Xalan ?
An interpreting version of XSLT.
Question 23 :
What is XHTML ?
An XML look-alike for HTML defined by one of several XHTML DTDs. To use XHTML for everything would of course defeat the purpose of XML, because the idea of XML is to identify information content, and not just to tell how to display it. You can reference it in a DTD, which allows you to say, for example, that the text in an element can contain < em > and < b > tags rather than being limited to plain text.
Question 24 :
What is XLink ?
The part of the XLL specification that is concerned with specifying links between documents.
Question 25 :
What is XLL ?
The XML Link Language specification, consisting of XLink and XPointer.