There is something very basic about CSS that I seem to be missing. Consider (

Given that there are no paddings or margins, what is the narrow white space between the two borders?

CodePudding user response：

This clearly is a rendering bug in Chromium-based browsers, as I was able to reproduce the issue both in Chrome and Edge at 250% zoom level, and in the code pen also at 125% zoom level (current stable version, MacOS 12.0.1, ARM).

I've raised an issue in the chromium bug tracker: https://bugs.chromium.org/p/chromium/issues/detail?id=1275530