What are some UX/UI rules-of-thumbs/principles that only people in the industry will know?

Hi all,

I'm building https://www.ezid.io/ where we help developers build passwordless signup and login for their websites.

I was hoping to get some insight into best UX/UI practices that are used in the industry i.e. 'assume your user is drunk'. (not sure if that even is one, but have heard of it)