Things like sitemaps or robots.txt have been known for decades and are ovbious by now. Here's the non-obvious stuff a webmaster should take care of.
- https://securitytxt.org/
- https://web.dev/change-password-url/
- webp with graceful fallback: https://serverfault.com/questions/630212/conditionally-serving-high-resolution-and-webp-images-with-nginx and https://alexey.detr.us/en/posts/2018/2018-08-20-webp-nginx-with-fallback/
Lastly, keep an eye on https://mozilla.github.io/standards-positions/