I found this article very useful: https://www.html5rocks.com/en/tutorials/internals/howbrowserswork/.
It’s about browsers, and how they work influences the applications we test in it.
And it’s very long, but worth of your time. I recommend reading the article, while you can skip the video in the beginning, because the article is more clear and detailed, and the video does not show all the images put in the article.