I'd like to propose for the list: Default heading styles should not have equal top and bottom margin. Headings should be closer to the content they label than to the content they are setting their content apart from. h1, h2, h3 should not have different styles. it's an anti-pattern that leads to broken accessibility
I came across it when reading Butterick's Practical Typography and it's possibly the lowest effort/clearest improvement guideline in the book.
Now I can't unsee websites that do it wrong.