You don't have access to the browsers 'zoom' from within a page (good thing!)
If it is Chrome only, you can use the "zoom: 80%" style on e.g. one of the top html tags.
As for all browsers, zoom can't be used (not supported). You then can use this (but it has not exactly the same effect as zoom):
-moz-transform: scale(0.8);-ms-transform: scale(0.8);-webkit-transform: scale(0.8);-o-transform: scale(0.8);transform: scale(0.8);
must be used together with for example here center top as point of origin:
-moz-transform-origin: 50% top;-ms-transform-origin: 50% top;-webkit-transform-origin: 50% top;transform-origin: 50% top;