CSS

I think there should be some sort of guidelines to help people decide whether or not to use web fonts.
Silky smooth interactions are critical for providing a natural-feeling application.
Grid Layout finally gives us the ability to define grids in CSS and place items into grid cells.
Over the last several years, the way I write CSS has transitioned from a "semantic" approach to something more like what people call "functional CSS.'
A simple morphing page transition effect where an SVG path gets morphed into another while the current page moves up.
You click a button and see a blue border telling you that you've clicked on something that you know perfectly well you've just clicked on.
We've all been there. Landed on a website only to be slapped with a modal.
Variable Fonts are still in active development, and the CSS Fonts Module 4 spec fully embraces them as the future of typography on the Web.
If you're a web developer, chances are you're going to have to write some CSS from time to time.
Sound is enjoying a renaissance on the web: the Web Audio API has great support, and most operating systems now ship with built-in narration tools.