When I write in the URL bar
javscript:document.getElementById("existing_elementid").innerHTML = "some text";
my browser doesn't change the innerHTML of the object but removes the whole page and set "some text" instead.
After that the source code looks like this:
<html>
<head></head>
<body>
some text
</body>
</html>
Edit:
whoa, i hope there's a better solution to this, but.. this seems to work.. (right on this stackoverflow page, in fact)
javascript:document.querySelector("#nav-questions").innerHTML="test";window.stop();