This was mentioned in chat:
https://www.256kilobytes.com/content/show/4399/get-these-dependencies-off-my-lawn-5-tasks-you-didnt-know-could-be-done-with-pure-html-and-css
A few that I found interesting:
Smooth scrolling
html {
scroll-behavior:smooth;
}
Accordion toggles
<details>
<summary>Sample Text</summary>
<p>Read this sample text.</p>
</details>
Progress bars
<meter value="4" min="0" max="5">Section 4 of 5</meter>
<br>
<progress value="69" max="420"></progress>
<meter value="0.69">69%</meter>