That’s exactly what collapsing margins are doing. It makes your div have 20px margin, not 40px. Imagine two blocks one above another. If margins were not collapsing - your divs would have 40px margin instead of 20 like you set.
> If I wanted a margin of a fixed size that doesn't dynamically change based on content
It does change based on browser width, that’s what most would want. From a designer perspective - the wider the element is - the more margin on top and bottom it should have for better readability. You don’t want your margins to be different for a list of various text items for example, it will look ugly. You want margins to be different based on a screen size.
Personally, I've not found collapsing margins a huge issue either way. It's just the way it is. There's 10 billion ways to add distance between two elements in the browser, so pick a solution and go with it.
So using width to compute a vertical margin is somehow a better solution? If I wanted a margin of a fixed size that doesn't dynamically change based on content, I would define it exactly using vh, or px.
> It's a great solution as well, imagine you have a div that you want to have margins on top and bottom of 20px. It makes perfect sense to collapse margins if you have say 2 of those div's running one after another, because you don't want them to have 40px margin, you only want 20px. So when there is another element that is not that div - you will still have your desired 20px margin.
I just want the code to behave as I instructed it. If I say this div should have 20px of margin on top and bottom, then it should. Don't go behind my back and say "we know better" and delete it. There seem to be plenty of ways to achieve only 20px on successive divs without resorting to what is basically undocumented behavior that explicitly contradicts what is programmed in the file. For example, I could just give all divs 20px of margin-top, and then style :last-child with 20px margin-bottom. This is almost certainly a better solution than resorting to collapsing, at least in my opinion.