Sizing Units

The Question the Unit Answers

A unit is not a formatting preference. It is a declaration of what happens to a value when the user enlarges text.

• rem — this value belongs to the text and must grow with it.
• px — this value belongs to the screen and must stay put.
• %, fr, ch, clamp(), min() — this value belongs to the available space.

Pick the unit by answering that question, not by picking a house style and applying it everywhere. Both absolutist positions — all-rem and all-px — produce defects, and they are different defects.

Zoom Is Not the Font-Size Setting

The common argument for px everywhere is that browsers already have full-page zoom, so the root font size can be ignored. They are separate controls serving separate people.

• Full-page zoom magnifies everything, layout included. It buys larger text at the cost of horizontal scrolling and fewer items per screen.
• The browser's default font size enlarges text and leaves the layout alone. This is what a user with low vision sets once, years ago, and never touches again. They will not switch to zoom because your product ignored it.