how to increase font size in UIWebView

I have 2 buttons – A- and A+

@interface
NSUInteger textFontSize;

- (IBAction)changeTextFontSize:(id)sender
{
    switch ([sender tag]) {
        case 1: // A-
            textFontSize = (textFontSize > 50) ? textFontSize -5 : textFontSize;
            break;
        case 2: // A+
            textFontSize = (textFontSize < 160) ? textFontSize +5 : textFontSize;
            break;
    }

    NSString *jsString = [[NSString alloc] initWithFormat:@"document.getElementsByTagName('body')[0].style.webkitTextSizeAdjust="%d%%"", 
                          textFontSize];
    [web stringByEvaluatingJavaScriptFromString:jsString];
    [jsString release];
}