Zoom Vs. Scale in CSS3

zoom is not standard css feature.

from MDN:

This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.

Use transform: scale(0.8) instead.

Complementary to Drkawashima's answer:

• zoom doesn't work in Firefox at all. See caniuse
• Once upon a time (fairy tale ends here sorry), zoom: 1; was the mighty declaration that helped to debug IE6. It conferred the element it was applied an internal "switch" to this browser named hasLayout (not a CSS property, just a concept like "clearfix" is). You'll find position: relative; zoom: 1; quite a lot in old projects

zoom does not work with css animations or transition propriety: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_animated_properties

Transform is more predictable than zoom across browsers.

Zoom affects positioning differently in different browsers.

example: position:absolute; left:50px; zoom: 50%;

• IE will not change the left value at all.
• Chrome will change the left value to 25px. Effectively it does do left = left * zoom. But DevTools Computed Values in DevTools will still display left: 50px, even though that is effectively not true due to the zoom.

Transform is handled the same way in all browsers (as far as I can tell).

example: position:absolute; left:50px; transform: scale(0.5)

• left would effectively be set to 25px in both Chrome and IE. (again, computed values will still not reflect this, it will display left:50px)
• To avoid changing the left value, simply use transform-origin: 0 0. That will ensure left is still 50px.

Demo: http://jsfiddle.net/4z728fmk/ shows 2 boxes where the small one is zoomed or scaled to 50%. Looks like this:

edit: img updated 2016-06-16 with Firefox (nothing had change in Chrome or IE since last time)