If the result of that calculation is 9px for you, your browser's default font is set to ~7px. That's even smaller than Chrome's "Very small" preset of 9px, and

If the result of that calculation is 9px for you, your browser's default font is set to ~7px. That's even smaller than Chrome's "Very small" preset of 9px, and significantly smaller than the normal default of 16px. I'd say that's on you.