Safari cannot load local resource into HTML page

Am doing some front-end local development on my Mac, and am running a local Jekyll server. Have created a local default.html file, and a local css file (/css/main.css) which is linked to the default.html file. Problem is that when I open default.html in Safari, it returns an error message saying "Failed to load resource; the requested URL was not found in this server. An error occurred trying to load the resource file:///css/main.css" I have shut down and restarted the Jekyll server, but get the same results; the main.css file will not load, even though default.html loads properly.

Both files can be seen in Finder, so there is no problem there.

Do I need to clear the Safari cache and try again? What else can I do so that the default.html can load the main.css file?