Learn the logic of great typography
Fun! But a heavy emphasis on doing things by eye. Personally I prefer the results of following stricter rules extracted from print typesetting. A few examples:- I recommend reading about [vertical rhythm](https://24ways.org/2006/compose-to-a-vertical-rhythm), which turns a lot of imprecise choice to exactly correct sizes.- I've never heard of there being a rule for how much space goes above a paragraph (this is a funny thing about web typography, since in print you indent the paragraph so there's no additional space). But I guess I'd consider it correct from a vertical rhythm standpoint to either use the line height or half of it.- Regarding vertical space between lines, what I've heard is you want to make the spacing between lines visually appear greater than the space between words. As far as I know this is just done by eye. (I.e., so the rule is to choose a leading that makes your space between lines appear larger than the space between words.)- I think there are exact header sizes that are better than picking an arbitrary font size that seems to look big enough. The trick I use here is to take the [traditional point scale](https://markboulton.co.uk/journal/five-simple-steps-to-bette...) and convert it to a ratio relative to 12 point. This is a way to bake historically refined font ratios into your design.- Personally, when choosing margins I'd use a [grid system](https://practicaltypography.com/grids.html), which will give you an exact gutter size. But admittedly this isn't too important for a single column of text.- There's simple rules that are missing here like make sure your width allows for 7-12 words per line, which [research](https://baymard.com/blog/line-length-readability) has illustrated improved readability.- Another good missing rule is never use any two font treatments on the same page that are too similar to each other.  This usually means having two things different between two font treatments, e.g., a different font size and weight for say a subheading vs. a heading.