evaluateJavaScript:completionHandler: Works on iPhone, not on iPad?

My app has HTML pages for content, and I have a button-triggered action to change the size of the text by using a set of functions that stepwise increases the WebKit webview text by changing the class of the document body (i.e."smallestText" = CSS webkit-text-size-adjust:80%) using the code below as the last step.

On the iPhone simulator, this works perfectly but on the iPad simulator the text size does not change even though the functions are called, the javascriptString is correct, and the _webview contains the web content.

I thought this might be a doctype version issue (most of the pages are HTML 4.01 strict) but changed it to HTML5 (!DOCTYPE html) with no change.

Any ideas?

 (void)refreshWebview {
    NSString *javaScriptString = [NSString stringWithFormat:@"document.body.className = '%@'", self.fontSizeName];    
    [_webview evaluateJavaScript:javaScriptString completionHandler:nil]; 
}