I use relative units (ems/rems) for font sizes and absolute units (px etc) for everything else such as margins/paddings/borders. I also set the html font size t

I use relative units (ems/rems) for font sizes and absolute units (px etc) for everything else such as margins/paddings/borders. I also set the html font size to 62.5% which makes the math easy (1rem = 10px). This ensures that if a user adjusts their browsers font size for accessibility purposes or uses dynamic type, only the text will scale. You don’t want margins/paddings to scale with font size (at least not linearly) otherwise the layout gets cramped/unusable very quickly. As others have mentioned CSS pixels have nothing to do with hardware pixels. So there are no issues with the differences in screen densities between devices.