The font-optical-sizing property automatically sets the font-size to the opsz - optical sizing axis of variable fonts that support optical sizing. This improves styling and legibility of fonts depending on font size because the font chooses a glyph shape that works optimally at the given font size. For example, the glyph contrast is improved in fonts in heading sizes when compared to the same font at body text size. See the spec link for an illustration of optical sizing.
Docs: https://developer.mozilla.org/en-US/docs/Web/CSS/font-optical-sizing