How to de- and increase the font size with WKWebView?

Swift 5 solution:

Implement WKNavigationDelegate and in viewDidLoad :

webView.navigationDelegate = self

then in didFinish :

func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
    let js = "document.getElementsByTagName('body')[0].style.webkitTextSizeAdjust='200%'"//dual size
    webView.evaluateJavaScript(js, completionHandler: nil)
}

The codes suggested by Inna used to work but stopped working from iOS10.3. I did some research and found that we need to make a slight change to it, just changing "%%" to a single "%".

So JS code should be:

let js = "document.getElementsByTagName('body')[0].style.webkitTextSizeAdjust='\(fontSize)%'"

I got this info from Apple Developer forum. https://forums.developer.apple.com/thread/51079