@thisisjofrank@mastodon.technology devdocs.io for sure
has basically all the quick info I need, no matter the language/framework im working with, and also works offline

And specifically for CSS, these amazing illustrated examples for flex and grid properties
https://css-tricks.com/snippets/css/complete-guide-grid/
https://css-tricks.com/snippets/css/a-guide-to-flexbox/