Despite being in IT and doing things in and around web-development for 20 years, my focus has been on System Administration for a long time. I 'know' HTML, Javascript, and CSS at a perfunctory level and can do all sorts of fun (but typically ugly) things in them, but modern 'front-end' development has changed a lot over the last 5-10 years, and I haven't had the time (nor inclination, to be frank) to keep up.
So, for others like me, here's two handy-dandy links that do a pretty good job of explaining not only what modern front-end dev looks like for CSS + JS, but how we got there.
CSS: https://medium.com/actualize-network/modern-css-explained-for-dinosaurs-5226febe3525
Javascript: https://medium.com/the-node-js-collection/modern-javascript-explained-for-dinosaurs-f695e9747b70
Having read them, I am now moderately enlightened, and I highly recommend them as worth the time to read. Doesn't mean I'm changing careers to be a front-end dev though :)