30 Seconds of CSS
Long-scrolling One Pager hosting a curated collection of useful CSS snippets you can understand in 30 seconds or less. You can find the updated non-One-Page site (Oct 2021: the redesigned to a multipage resource you can find here.
This website has unfortunately redesigned or gone offline, so I have removed the link to it. The screenshot below hopefully preserved enough of the design but if you are really keen to inspect further, try this link on the Archive.org website. FYI: the site was first featured on 09 April 2018.