The site the glossary's modern CSS entry is named after, and the best worked-example set for it: each article states the old problem, shows the old workaround, and replaces it with the modern property — aspect-ratio for the padding hack, gap for the negative-margin grid, :is() and :where() for the selector lists, custom properties for the theme switch. Eckles writes for developers who already know the workaround, which is the audience that needs to be told it can stop.
SmolCSS is the companion: the same ideas as snippets — the responsive grid, the flexible sidebar, the aspect-ratio card — each under a screen of CSS. Style Stage is the Zen Garden idea again, which is this site's own theme model, run as a community. All free; there is a paid course beside them.