How can I make it so that my HTML5 website has its own browser zoom setting

How can I make it so that my HTML5 website has its own browser zoom setting, like agar.io does?

For example, if I zoom in 200% on this stackoverflow page, then navigate to agar.io, the zoom will be reset to 100%. Then if I return to this page (or any other page), the zoom is back at 200%.

Likewise, if I have multiple tabs open, zooming the agar.io tab will affect only that page's zoom setting and zooming from any of the other tabs will not affect agar.io's tab.

I've searched and searched but can't find any documentation (or even questions) about this.

P.S. - I've only tested this behavior on new Firefox and Chrome browsers, on my MacBook..