font-size-adjust: ex-height 0.5 instructs the browser to scale the font such that the letter "x" is exactly half of the box.

A use-case for font-size-adjust I find much more compelling is that you probably are going to use several fonts on a web-page. And you also might change fonts in the future. And they will have different intrinsic size because that’s how the things are. Part of the mess is avoidable by pinning the meaning of font size.

0.53 is the invariant ratio for Helvetica.