As a concise overview over the current state of all things front-end (The Front-End Practice, Learning Front-End Development, Front-End Development Tools), Cody Lindley wrote the Front-End Developer Handbook 2018.
Auto-Hiding Navigation in CSS and jQuery | CodyHouse
Structuring CSS in large projects (Medium)
How Good Are Your HTML and CSS Comments? (Sitepoint)
Accessible, Simple, Responsive Tables (CSS Tricks)
TestMyCSS.com
Link: TestMyCSS.com
Editing Images in CSS (The Dot Post)
CSS Protips (GitHub)
Link: CSS Protips (GitHub)