Invocation for localhost testing, with http server running in correct directory:
Results:
HTML source
<x3d-canvas url=' "HelloWorld.x3d" '></x3d-canvas>
works both on localhost also remotely via http/https.
HTML source
<x3d-canvas url=' "http://localhost:8000/HelloWorld.x3d" '></x3d-canvas>
only works on localhost.
Once an http connection is first established to
this html page,
then the html source file does not need a http://localhost:8000/ prefix to retrieve the .x3d model.
Similarly, no prefixes are needed within the .x3d model source.
Avoiding that prefix in source means that fully developed pages and models, once debugged and working on localhost,
can be published directly to an external http server without further modification.
Lesson learned: do not include http://localhost:8000/ within the .html or .x3d source.