style: Make zoom: 0 mean the same as zoom: 1.

This matches the WebKit implementation, and is clearly a violation of the rules
we generally use for ranges in CSS.

But it seems to be depended-on legacy behavior, see the linked WebKit bug, this
bug, and bug 1593317.

Differential Revision: https://phabricator.services.mozilla.com/D51539

components/style/properties/shorthands/box.mako.rs

@@ -464,11 +464,13 @@ ${helpers.two_properties_shorthand(
         };
 
         // Make sure that the initial value matches the values for the
-        // longhands, just for general sanity.
+        // longhands, just for general sanity.  `zoom: 1` and `zoom: 0` are
+        // ignored, see [1][2]. They are just hack for the "has layout" mode on
+        // IE.
         //
-        // FIXME: Should we just do this for the "normal" case? Seems weird
-        // either way, so maybe not?
-        Ok(if zoom.get() == 1.0 {
+        // [1]: https://bugs.webkit.org/show_bug.cgi?id=18467
+        // [2]: https://bugzilla.mozilla.org/show_bug.cgi?id=1593009
+        Ok(if zoom.get() == 1.0 || zoom.get() == 0.0 {
             expanded! {
                 transform: Transform::none(),
                 transform_origin: TransformOrigin::initial_value(),