I'm STILL learning HTML and CSS after 20 years. Constantly surprised by new uses or semantics, or rediscovering a little known attribute or selector. One that comes to mind is background-image is an animated GIF with background-size: cover making it look like the page text is on the inside of a car window as its driving around.

It took a while for everything to 'click' and understand how DOM/JavaScript/CSS can really jive together. Using a framework is easier than understanding how something like Bootstrap works by examining the elements, CSS and scripts that all work together. I know there's always going to be more to learn, like any art form.