One of my favorite pages to explore on MDN is the directory of Web APIs. I barely know what many of them are, but it's interesting to learn about and test out newly available browser features and what may be lurking on the horizon.
A couple relatively new features are the MutationObserver and IntersectionObserver. I've used both to solve different problems in recent projects of mine: to be notified when an element is added to the DOM asynchronously (mutation) or to be notified when an element comes enters the viewport (intersection).
What WebAPI captures your interest?