When the zoom level is adjusted, I've noticed that the margins of list items don't scale correctly. Instead, they seem to stay the same size for a few levels of zoom before suddenly jumping to the next correct value. This might be because browsers are rounding off the margin values, causing 1.1px and 1.4px to still display as 1px until there's a noticeable shift.
You can see this behavior on a webpage like:
Pay attention to how the 'Take a look' button and thumbnails below it change when the page is scaled.
UPDATE: Further testing reveals that zooming works correctly at 50%, 100%, 150%, 200%, etc., but not in between those levels. This suggests a possible rounding issue within the browser.