ScaleWebView.swift

override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
    super.traitCollectionDidChange(previousTraitCollection)
        
    let referenceSize = UIFont.preferredFont(forTextStyle: .body, 
                                             compatibleWith: .init(preferredContentSizeCategory: .medium))
    let newSize = UIFont.preferredFont(forTextStyle: .body)
 
    let percentIncrease = (newSize.pointSize / referenceSize.pointSize) * 100
    let js = "document.getElementsByTagName('body')[0].style.webkitTextSizeAdjust='\(Int(percentIncrease))%'"
    webView.evaluateJavaScript(js) { _, error in
        if let error = error {
                print(error.localizedDescription)
        } else {
                print("success!")
        }
    }
}