element: Self is used in multiple places in the stub for etree._Element. Here's a typical example:
https://github.com/abelcheung/types-lxml/blob/main/lxml-stubs/etree/_element.pyi#L97

This produces over-narrowing of the type when used with custom element classes, like this:

class Frombus(etree._Element):
    def add_child(self):
        self.insert(0, element_that_is_not_a_frombus)
# --------------- ^^^ Argument of type "NotAFrombus" cannot be assigned to parameter
#                     "element" of type "Self@Frombus" in function insert

This is because Self takes the type of self (Frombus in this case) and only wants to allow elements of the same type to be inserted.

I think the solution is to use "_Element" in most (or maybe all) places where Self appears within the _Element stub, which I expect was the original intent (that element can be any subtype of _Element). If you add from __future__ import annotations at the top of the file I believe you can leave off the double-quotes.

This works as intended when I patch in these changes locally.