Explanation:

Both of the above boxes are positioned one half physical pixel away from the top edge of the page.

The left box is positioned 1 full physical pixel away, then the page is scrolled by a half pixel. If fractional-scroll-offsets is enabled in Chrome, the scroll offset will actually be half the physical pixel.

The right box is fixed and positioned one half pixel from the viewport top.

Observe the top edge of each box (you may need a screen magnifier or to take a screenshot and observe zoomed in using an image viewer). You should see that the right box paints using blending so that the top edge of the box is purple. The left box, even though it is positioned in the same place, does not paint using subpixel precision so it is snapped to the first row of pixels.

Here's a zoomed picture of what you should see