Pixel in CSS

CSS2 recommended 90ppi as the reference pixel, but CSS2.1 recommends 96ppia measurement common to Windows machines and adopted by modern Macintosh browsers such as Safari. This means when I display 96 pixel in my screen, it should be 1 inch length in physics. But if my screen is about 110ppi, so 1 inch in css is going to less than 1 inch in my scree. But how about printer, it normally print 600ppi. It actually first convert pixel in screen to inch in scree, then print the length in inch in printer, and print the pixel by timing 600ppi to length in inch.