If the given URL is invalid, no error will be reported - the browser will simply attempt to open it anyway.
Much like the
<a> HTML element, the URL is treated as a relative URL if it doesn't start
with “http:”, “https:”, or another such protocol. This means that if your story file is
hosted at “http://www.example.org/story.html”, then
(open-url: "page2.html") will actually open
the URL “http://www.example.org/page2.html”.