Hmm the link has changed, here's the new one if anyone else is looking for it: https://pilabor.com/blog/2021/03/html-and-css-tricks/

That page also has some tips I didn't know.

> <a href="#!">

(don't jump to page top on click)

It isn't anything special like that, it's trying to jump to an element with id="!" but not finding one. Any gibberish after the # that doesn't match something on the page would work.