Research: What triggers an iframe's native 'load' event in different browsers?

E.g. when:
- iframe is created and appended to DOM
- doc.open() is called
- doc.write() is called
- doc.close() is called
- doctype is written
- iframe src is changed
- iframe src attribute is written as it already is
- moving iframe element in the DOM
- moving parent of iframe element in the DOM
- iframe element display:none => display:block
