WebKit now supports full page zooming on Mac and Windows. Because this feature involves new WebKit API, it’s not something you’ll see in nightly builds by default (since the Safari used with the nightlies calls the text zoom API). I have added a debug preference, though, that will cause the text zoom methods in WebKit to do full page zoom instead.
To try out full page zoom in a nightly, you can type the following in a terminal window:
defaults write com.apple.Safari WebKitDebugFullPageZoomPreferenceKey 1
We encourage people to try out full page zooming and file bugs on the feature.