How to change UIWebView or WKWebView default font

You can use your UIFont object (so you can set it and modify more easily), but wrap the HTML in a span instead; font tags have been deprecated since HTML 4.01.

UIFont *font = [UIFont fontWithName:@"GothamRounded-Bold" size:14];

Assuming you already have the NSString *htmlString created, you can use the font's properties:

htmlString = [NSString stringWithFormat:@"<span style=\"font-family: %@; font-size: %i\">%@</span>",
                                        font.fontName,
                                        (int) font.pointSize,
                                        htmlString];

Alternatively, you could just supply the values instead of using a UIFont object:

htmlString = [NSString stringWithFormat:@"<span style=\"font-family: %@; font-size: %i\">%@</span>",
                                        @"GothamRounded-Bold",
                                        14,
                                        htmlString];

Just prefix a <font face> tag to your string before loading into the webView.

NSString *body = [plist objectForKey:@"foo"];
NSString *htmlString = 
    [NSString stringWithFormat:@"<font face='GothamRounded-Bold' size='3'>%@", body];
[webView loadHTMLString:htmlString baseURL:nil];

You could try to set the font by injecting a line of JavaScript like this:

[webView stringByEvaluatingJavaScriptFromString: @"document.body.style.fontFamily = 'GothamRounded-Bold'"];