XHTML has two DTDs:
a so-called Strict DTD, which is content-oriented,
and another one, much more compatible with traditional HTML, called the Transitional DTD.
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.