SAX limits

You cannot back up in the parse.

You must control the context. In other words, you must be able to grab the data you need as it goes by, while ignoring the data you don't need.

There is no structure in memory to do in-place updates.

The order in which you write the XML is important and you cannot modify the XML once it has been written.

It is possible to create an XML document that is not well-formed.

You do not know if an XML document is well-formed or conforms to a schema until you have parsed (and processed) the entire XML document.