Setup Open your browsers dev tools to their network tab, filtered to 'styles.php' Disable the browser cache and, if possible, enable the "Preserve log" function to assist you Generic tests Refresh the page Purge your caches and refresh the page (or press the "Purge all caches" link) Confirm that there was a single request to styles.php which returned straight away Confirm that the page fully rendered and that the call to styles.php looks to have finished Navigate to another page Confirm that there was a single request to styles.php which returned straight away Confirm that the theme looks as anticipated, including language direction RTL Langauge Switch to an RTL language Run the Generic tests again Switch between LTR + RTL langauges without a cache purge, confirming that you get the correct stylesheet each time Alternative theme test Repeat all of the above tests with the More and Clean themes. Brownie test Add error_log calls to theme/styles.php around the fallback handling and: see if you can understand how it works ensure that if one fallback process starts the queueing process, that another just skips the process entirely because it's unable to get the lock Changes test Make a change to something obvious in your theme (e.g. colour) and save changes Confirm that there was a single request to styles.php which returned straight away Confirm that the page fully rendered and that the call to styles.php looks to have finished Wait a while for the theme to generate and navigate to a different page Make a change to something obvious in your theme (e.g. colour) and save changes Confirm that the change was present Confirm that there was a single request to styles.php which returned straight away