How can I scale the content of a WebView?
Or you can try the following:
- (IBAction)takeZoom:(NSSlider *)sender {
WebView *aWebView = [[window.contentView subviews] lastObject];
NSView *clipView = [[[[aWebView mainFrame] frameView] documentView] superview];
float scale = pow( 4, [sender floatValue] )/(clipView.frame.size.width/clipView.bounds.size.width);
[clipView scaleUnitSquareToSize:NSMakeSize(scale, scale)];
[clipView setNeedsDisplay:YES];
}