What happened to 5.1.4 in version 4.1.0?

Before 4.1.0, the minimum uppercase letter size is defined by the distance from the hardware.

Now in 4.1.0, the physical text size is calculated by the x height of CSS pixels. On native hardware, there might be no CSS involved, and the hardware might use device pixels which can be very small. The requirement is now that the minimum text size can be increased to 16 CSS pixels x-height.

Note that there is no technical way in CSS to specify the x-height, only the font-size. This is therefor almost impossible to really verify or test.

In addition, where the previous guidance was that at 500mm distance the uppercase letter should be 6,1mm, the new requirement proposes an x height of x-height (mm) = D (mm) × CSSpx / 2690 = 500 × 16 / 2690 = 8.000/2690 = 2,974. So about 3 mm. As the x height is usually about two thirds of the height of a letter, give or take, this would make the zoomed in minimum font as about 4.5 mm. This is way smaller than the previous requirement.

I see there is note 2, which says “This is the smallest font size - not the recommended font size for body text which should be larger.” but there is no guidance around this.

The introduction of x-height makes little sense here, and even if you want to base it on the CSS pixels, you could define it as a font size, with the requirement of 24 pixels, assuming the x-height is roughly 2/3 of a font.

I understand that the x-height is important to readability, especially in relation to the letter height, but using it here feels unnecessary and complicated.