Invalid attribute names

Does anyone know if an xml well-formedness checker needs to check for invalid attribute names or not?

 

From what I have found so far:

A tag attribute name must begin with a letter (i.e., upper or lower case A through Z) and can only consist of letters, digits (i.e., 0 through 9) and the hyphen, period, colon and underscore characters (i.e., "-", ".", ":" and "_" respectively).

 

However, this simple test at, ( http://www.xml.com/pub/a/tools/ruwf/check.html) passes this simple xml file

 

So I guess my question is whether or not a xml editor is required to even check this or not? The fact that a colon is the first character in the attribute name seems to be illegal.

 

Thank you very much