1. Container queries are a big topic that’s part of laying out.
2. Cascade layers allow the organization of CSS files into hierarchical layers baking in specificity using @layer. Some layers in CSS already exist. The user style sheet built into the browser is one layer, below the layer of the regular CSS file. This is widely supported in all browsers.
3. Relative colors
4. Scoped styles using @scope. This is very new and not yet available in all browsers although only FF is missing this currently and even that can be enabled about:config.
5. Style queries use container queries to check for CSS rules. Not yet available in all browsers. In fact only Chromium browsers and even then only with CSS custom properties.
6. Relational selector has(), known as the parent selector, is widely available in all browsers and has some interesting possibilities.
7. Anchor positioning allows you to link elements like popovers to other elements.

More stuff

Those listed below may or may not be added in the near future. There is so much new CSS these days that it’s hard to keep up. Some things can take years to make it into all browsers whilst others may not be that useful most of the time.

1. CSS if statements
2. CSS functions
3. Mixins
4. scroll-target-group