HTTPS test suite.
Lighthouse is chrome's built in page analyzer, but you can run it headless fairly easily.
Seems like a good proof of work captcha library.
Demonstration of basically every html element and it's usage.
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">
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">
Lossy PNG compressor.
Lossless PNG compressor.
Extremely effective JPEG compressor.
Dithering can be used to save significant amounts of bandwidth and or to create an interesting aesthetic.
exiftool -all= image.jpeg