1. The XHTML Strict DTD versus the XHTML Transitional DTD

XHTML has two DTDs:

XMLmind XML Editor supports both DTDs using the two different configurations. However, if you have created your XHTML document using an editor which does not add:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

or

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

at the top of the document, XXE will fail to recognize your document as being an XHTML one. In such case, using a text editor, you need to add the proper <!DOCTYPE> to your XHTML document.