What are events in CSS history that changed the way you write CSS?

For me for example:

- when Snook published SmaCSS
- when @beep wrote the ALA post about media queries
- when I learn about BEM
- when I started to use custom properties

PostCSS Preset Env - CSSTools

PostCSS Preset Env lets you transform modern CSS so most browsers can understand, determining the polyfills you need based on the browsers you want.