See https://crisal.io/words/2020/06/13/rounding-borders.html

At least the rounding up to device pixels happens consistently at computed value time:

• https://webkit-search.igalia.com/webkit/rev/357f5ddb37eef4235a7ca33367f9833b4204705e/Source/WebCore/style/StyleBuilderConverter.h#297
• https://source.chromium.org/chromium/src/+/master:third_party/blink/renderer/core/css/resolver/style_builder_converter.cc;l=1077;drc=6993ed8e5cdcfbae4e5ff7891ddfb4de128dd846

Shipping Gecko and WebKit (my post was wrong, Blink and WebKit diverge, will fix) also floor to dev pixels at computed value time. Blink floors at rendering time, which I think is more unfortunate.