RTFM site delayed style load in Internet Explorer 11

In Internet Explorer 11, with a site built using RTFM, the CSS for the menu sidebar is delayed. The pages load in a very spasmodic fashion.

There are a lot of '403 - forbidden' error messages showing for SCSS files.

Is this a known issue? Are there any solutions?

I notice that this is a problem with the learn.getgrav.org site itself!

Observe how (with IE 11 only) each time you select a page from the menu, the menu disappears and the page momentarily takes over the entire screen