Great episode, Chris. I like how you said you don't really need to test CSS styling but instead focus on testing the higher level functionality of your mailers. That way, a developer doesn't go down a rabbit hole writing too fine grained tests.
Also, the last point about ActionMailer::Base.deliveries makes me wonder what the difference is between that and assert_emails.
Yeah, I think I know what you mean. querySelectorAll returns an object but since you can do something like object to get the first element in the object, it looks like you might be working with an array. Great point.