This problem started a few weeks ago.
Whenever I navigate to any page at
http://clhs.lisp.se — for example this one:
http://clhs.lisp.se/Body/m_w_open.htm — my browser doesn't display the page normally, but instead attempts to download the page as an HTML file.
This happens in Chrome, Firefox, Brave, Vivaldi and Opera; but not in Internet Explorer or Edge. It used to work fine in all browsers.
Why does it happen, and how can I make it work normally? Using IE or Edge is not an option.