I understand this, but let's be more concrete: when you use CSS to design for print (that is, in general, to produce a PDF), you are using a specific implementa

I understand this, but let's be more concrete: when you use CSS to design for print (that is, in general, to produce a PDF), you are using a specific implementation (e.g., Firefox will be used to produce the final PDF from your HTML and CSS, or Inkscape will be from your SVG and CSS). Given an implementation, you get a determined rendering, and it is this specific one that you need to be able to tweak to your needs. Same goes for application UI design: a given GUI framework comes with its own implementation of CSS rendering and it is its specific output that developers (or rather, theme designers) need to be able to have precise control over. In both cases, even if the standard does not precisely states what property values must give what rendering, you get something using the tools you have and you may want to tweak details to your taste using those same tools (even if only by trial and "error", but for that you need the option to fine tune the properties you want to play with). Again, I'm not saying CSS is coherent nor that this is a good thing in the absolute sense, as I already said, I agree with your feeling, I'm just saying that I understand where the need of more precise control/tweaking over details that is sometime in the standard comes from.