The font-size-adjust CSS property adjusts the font size based on the height of lowercase letters rather than that of uppercase letters for consistency in readability. However, there have been voices that the adjusting metric is obscure and impractical. This new two-value syntax reflects such voices by allowing developers to specify a font metric for adjusting the size. This feature is standardized in CSS Font Module Level 5.
This feature is in web designers' demand and one of the focus areas for Interop 2024 [1]. [1] https://github.com/web-platform-tests/interop/issues/541#issuecomment-1846090776
Samples: https://developer.mozilla.org/en-US/docs/Web/CSS/font-size-adjust