Use jQuery actions on scroll, for example to show a header only on scrolling up, or changing section color when scrolling past a specific element.
Set a dynamic class to a section/widget from an ACF field value. If value is empty, no class is added. Very handy with checkbox to use for conditional CSS with (custom) posts.
Add CSS to the WP-Admin dashboard backend, for general styling or to hide elements for specific users
Use php to add body classes based on any condition to use for hooking into for CSS styling
Forward a visitor of a page to another url x time after landing, and show the countdown
Hide specific posts and/of post-types from search results.
Check with JS if the URL that the visitor came from when landing contains a certain string