Hallo Julius,
If action is the empty string, let action be the document's address of the form document.
vielen Dank fürs Heraussuchen!
Interessant an dieser Stelle:
The action of an element is the value of the element's formaction attribute, if the element is a submit button and has such an attribute, or the value of its form owner's action attribute, if it has one, or else the empty string.
Aber action=""
ist nicht erlaubt.
Bis demnächst
Matthias