1/96th an inch based on what the browser/system report the pixel density to be. There is no man coming out of your computer with a ruler to verify the reported value, so you're free to use a scaling value that isn't true. Zooming the browser or changing the system scaling will change the reported size of the pixels to the renderer.

Don't take my word for it, it's how it's defined in the CSS spec: https://www.w3.org/TR/css-values-3/#absolute-lengths