ViewController.swift

import UIKit
import WebKit

class ViewController: UIViewController, WKUIDelegate, WKNavigationDelegate {
    var webView: WKWebView!
    
    override func loadView() {
        let webConfiguration = WKWebViewConfiguration()
        let contentController = WKUserContentController()
        let js: String = "var h1s = document.querySelectorAll('h1'); for (var i = 0; i < h1s.length; i++) { h1s[i].style.color = 'red' };"
        let userScript = WKUserScript(source: js, injectionTime: WKUserScriptInjectionTime.atDocumentEnd, forMainFrameOnly: false)
        contentController.addUserScript(userScript)
        webConfiguration.userContentController = contentController
        
        webView = WKWebView(frame: .zero, configuration: webConfiguration)
        webView.uiDelegate = self
        webView.navigationDelegate = self
        view = webView
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let myURL = URL(string: "http://localhost:3000")
        let myRequest = URLRequest(url: myURL!)
        webView.load(myRequest)
    }
}