Why do webpages sometimes appear as plain HTML before a refresh

This happens when the CSS (stylesheet) for that page is not loaded properly. Now, why that happens is a more complex issue. You'll need to do this to troubleshoot that issue:

1. Open Tools | Developer Tools from Chome menu.
2. Switch to Network tab there
3. Keep that window open, browse as usual. Once you see that phenomenon again, go to that Developer tools window, and see which file was not loaded properly and why.

It could be a DNS issue, or some transparent proxy acting up, or some other problem. My own company's CDN (content delivery network) provider had issues yesterday, and it manifested itself in a similar way to what you describe here.

Since you say the issue is transient, and goes away after a refresh, it's rather hard to troubleshoot.

There is an old problem with .css files which appears on some browsers depending of how are loaded or declared the .css documents containing the style rules.

This is called the "Flash of unstyled content (FOUC)". This concept has even a dedicated Wikipedia page with links to additional sources of information.

According to your screenshot, this seems to be the problem that you are suffering. The problem can only be corrected doing the necessary changes in the page source, so you can't solve it.

I'm not 100% sure, but perhaps you're having issues downloading the javascript or CSS for the site. Sometimes (for big companies) the CSS & javascript are stored on a 3rd party "CDN" (content delivery network)... which frequently end up on "ad-blocking" lists by accident. Have you tried disabling ad-blocking software? Perhaps you have some nasty-bug on your machine... and it's redirecting web-traffic.