ETag value must be a quoted-string

(difficulty: easy, requires to use a network sniffer) Open a 3rd party network sniffer e.g. Fiddler and start capturing the network (HTTP) traffic. For Fiddler, press F12 Open your preferred browser e.g. Chrome and cleanup its cache Point the browser to the home of your Moodle instance Check each captured response: HTTP Status must be 200 and in the Response Headers , the ETag value - available for any CSS / Image / JS provided by Moodle - must be surrounded by quotes Redo a browser request to the same page: now any of the above response will be provided by the browser cache, the HTTP Status must be equal to 304