You can use media queries to style elements based on features of the browser viewport, for example, min-width, max-height, or orientation. With container queries, you can now do the same but with any parent element. Instead of the viewport, you can now listen to properties and features of a containing element.
Most elements have no preferred aspect ratio. On day 42 I’ve explained how you can use the aspect-ratio property to define a ratio for these elements. Replaced elements like <iframe>, <video>, <embed>, or <img>, on the other hand, have an intrinsic aspect ratio. This means that you don’t have to define one using the aspect-ratio property and they will scale naturally.
On day 9 I’ve talked about the inset shorthand properties inset, inset-inline, and inset-block. I don’t believe that I will need those often, but inset can come in handy when you want one element to fill another element entirely.