"Static website: very secure, very low server load, very fast page loading" Barry Kauler (of Puppy Linux fame...)
Use a font available in all browsers, do not import a font family
Avoid images, use only webp for images (writing this in 2020). Choose the correct image size.
Avoid Video-backgrounds, Carousels and other gluttons.
Keep CSS small, only a few kilobytes, just for what's needed. Internal in the head (<style></style>) section is then the best solution? [Edit:] Only true for one-page-sites. In multi-page sites like this one, the pages get bigger, so slower to load, the amount of work to change some style element becomes frustrating and last but not least with an external CSS file, a browser will cache that, making this solution a clear winner. (as so often, thank you Stackoverflow!)
Even an already very small CSS file can be minified, making it about 60% of the original. This is of course of very little importance (e.g. minifying 2.4k to 1.5k..), compared to optimizing an image. But it's nice to know it can be done.
Mobile First design --> One column for all. Natural, vertical information ... ( "Start Mobile First and Stay There!" - Heydon Pickering.)
Avoid every form of "middlemen" (cms, libraries, CDN, Google stuff, SSG, Bootstrap, etc..) (Not sure about CDN... Sure it's an extra dependency, it has its own unknown power consumption, it can fail and potentially make your site more vulnerable, but it has also been likened to "buying locally", avoiding long distance transfer. I don't know the definite answer to this one yet. *2021: I am now certain CDN is the way to go, it's superior in speed, ecological footprint and stability, compared to traditional internet)
Navigation (menu) on its own page..the hamburger menu loved by vegetarians ;)
Pure html and css and..Content first...
Always be aware of people on a poor internet connection, or people having to use an expensive data contract, or those living off-grid on a small solar panel and in general all those not living in the Wealthy Western Web.
Do not use social media buttons
Do not use a commenting system
Forget about SEO and all that comes with it. Do not worry about how your site ranks according to Google. Just express what you want, the way you want. Nothing more is needed. The right people will find your site. Good wine needs no bush!
If you need a little more than the most basic design and reponsiveness, Flexbox is the way to go. Grid is another great candidate. Lightweight, powerful, easy to learn. Personally I'm not convinced that I need it..but maybe I'm a weird exception ☺.
This list is just a start. I would be very grateful to receive your good ideas! Use the Contact page for that and if I like your proposal, I'll blog about you(r idea), only after your permission of course..! ❤️