Question about xml mention domain

After putting a picture of my self-designed <XMLennial/> T-shirt on FB, it was brought to my attention that this element name is invalid, as it starts with “xml”. I checked this on W3schools and the remark seems to be correct, unless there have been changes to the standard or its interpretation. Any tag name is legal except when it starts with either ‘xml’, ‘XML’, ‘html’ or ‘HTML’.

What does this mean for the tag names in the XML mention domain? If these are illegal against the XML syntax we have a problem, which would have to be corrected for DITA 2.0.

Kind regards

Jang F.M. GraatSmart Information DesignAmsterdam, NetherlandsCell: +31 646 854 996