I didn't say 'regulatory' tests, but researchers, over time have done extensive tests with varied groups of people to find out how readable different fonts, colours and designs are. This has led them to develop guidelines and advice on website design.
Anyone who uses the internet will know how some websites are easy to read and navigate while others you give up on because either they are difficult to read or difficult to navigate.
The Kindle font, Bookerly, was specifically designed to be easy to read on that medium and will have been extensively tested with all kinds of readers before being officially launched. I changed to it when it was launched to give it a trial, and then stayed with it because it was easier on the eye. Obviously in these situations you will not find 100% of readers like it, but the majority do.
I have always been fascinated by verbal graphic language - putting the language we use in speech onto paper - and how different designs can make the same text work in different ways and how our ideas of good design, while changing over time, still obey the same underlying rules to do with readability and engagement.