Are IFRAMEs inherently unable to display file:/// URLs?

Yes, the browser and webserver are on the same machine!

Whenever I try to embed a HTML file from outside of the WWW root in an iframe, thus using the file:/// URL syntax, it is ignored. No error message logged or anything. Just nothing is done whatsoever.

If I change the path from file:///C:\blablabla.html to ./blablabla.html, and put the file in the WWW root, it will display the HTML page in the iframe. So it’s not some kind of fundamental issue with displaying iframes or anything. I have tried both with and without URL-encoding the C:\... part.

Are IFRAMEs inherently unable to display file:/// URIs, even though the MozDev page mentions no restrictions whatsoever for the src attribute? https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe