The following table illustrates table errors. Mozilla's (0.8.1) XML renderer misaligns the second row relative to the first. Amaya (4.2.1) fails to right-align the first column.
Cell 11
Cell 12
Cell 21
Cell 22

If, however, Mozilla perceives the file to be HTML (not XHTML) as it does if the DOCTYPE is removed from the top and the file is loaded from a local disk, then Mozilla renders the table correctly. Presumably this shows that the HTML parser is not broken in this way.
When served from a webserver. Mozilla seems to have 3 possible behaviours.
1. If the file has an XML DOCTYPE and an extension .html the table is broken and possesses shaded borders.
2. If the file has an extension .xml, regardless of whether or not it has the XML DOCTYPE, the table is broken, and the borders on the table are rendered as single lines rather than shaded.
3. If the file has extension .html and NO DOCTYPE, the table is correctly rendered.

The first three lines of the "xmltab" version of this document are:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
	   <html xmlns="">

The "htmltab" version does not have the first two lines. (The DOCTYPE is removed).
There is no difference whatever in the ".html" and ".xml" versions. They are logical links to the same file.