I am completing a migration of a Drupal 7 website to Drupal 9 (which will immediately move to Drupal 10). The pure CSS parallax scrolling that I implemented in Drupal 7 works during page loading in Drupal 9, but as soon as the page has completed loading, the parallax scrolling stops working.
I have examined each of the elements of the CSS and explored other possible values.
I referred to the pure CSS tutorial here: https://keithclark.co.uk/articles/pure-css-parallax-websites/ which approaches the effect differently, so I would like to avoid re-writing all the HTML if possible.
I made a JSFiddle entry with my HTML and CSS to show that it works outside of the Drupal environment.