I have an iframe located just one directory away:

  <iframe id="menu" src="./menu/menu.htm" title="Menü"></iframe> 

Goal is to share the same menu among multiple pages. This iframe should be able to communicate with its “parent”.

Unfortunately, I run into the “cross-origin” error.

This happens even when all files are local and the parent is run from the browser as
file:///C:/Users/..../testiframe.htm

When specifying the iframe src with the same protocol and full path file:///C:/Users/...
I get the same error complaining about ‘null origin’.

BTW: The exception happens when running a script in the iframe with e.g.

console.log(top.location);

and has the full text

“Error: Failed to read a named property from ‘Location’: Blocked a
frame with origin “null” from accessing a cross-origin frame.”