What can we do better?

[RobinK]

RobinK. That is what I did and it works fine.  I am trying to figure out why it happens in the first place as it seems widely variable to who and when it occurs.

I'd start with the broken SSL setup. If you alter the address from http://www.tost3.com/ to https://www.tost3.com/, the error will reproducibly occur. Any browser that defaults to https:// (someone mentioned the DuckDuckGo browser which, if memory serves, uses https everywhere) will run into trouble. Get rid of the unencrypted connection and you should be good to go. Me, I'll delete some cookies now to get back the CSS files...