For those curious, in WebKit, this stems from the use of the LayoutUnit (https://github.com/WebKit/webkit/blob/main/Source/WebCore/pl...) for most computed length values. LayoutUnits use a fixed point representation where the smallest unit is 1/64 of a pixel. https://trac.webkit.org/wiki/LayoutUnit is a bit old, but has some good information on the topic.

This 2^25-1 pixel limit makes perfect sense - with 1/64 pixel precision, that's exactly 2^31-1 layout units (the max value of a signed 32-bit integer).

its the same in chrome I think

Blink only forked from WebKit after layout units settled at 1⁄64.