testing tools

https://observatory.mozilla.org/

HTTPS test suite.

https://github.com/GoogleChrome/lighthouse/blob/master/docs/headless-chrome.md

Lighthouse is chrome's built in page analyzer, but you can run it headless fairly easily.

captcha

https://git.sequentialread.com/forest/pow-captcha

Seems like a good proof of work captcha library.

https://matthewhowell.github.io/reasonable.html/

Demonstration of basically every html element and it's usage.

mobile

Inform mobile devices that they should just render your site normally. Hilariously this isn't even part of the "web standard", but it also kinda is since it's implemented everywhere.
<meta name="viewport" content="width=device-width, initial-scale=1">

images

lazy loading

Lazy loading is generally frowned upon. For people who like to load up pages before going somewhere with worse internet they may accidentally be unable to view your whole page. Instead you can use the decoding async attribute so that image decoding can be defered.
<img src="image.png" loading="lazy" alt="…" width="200" height="200">

optimization

https://pngquant.org/

Lossy PNG compressor.

https://github.com/shssoichiro/oxipng

Lossless PNG compressor.

https://github.com/tjko/jpegoptim

Extremely effective JPEG compressor.

dithering

Dithering can be used to save significant amounts of bandwidth and or to create an interesting aesthetic.

strip metadata

exiftool -all= image.jpeg