Sample code for "Injecting JavaScript Into Web View In iOS" (https://swiftsenpai.com/development/web-view-javascript-injection/)

webview-inject-javascript-full.swift

import UIKit
import WebKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Configure the web view for JavaScript injection
        configureWebView()
    }
    
    private func configureWebView() {
        
        let css = """
        html, body {
          overflow-x: hidden;
        }

        body {
          background-color: #333333;
          line-height: 1.5;
          color: white;
          padding: 10;
          font-weight: 600;
          font-family: -apple-system;
        }
        """
        
        // Make CSS into single liner
        let cssString = css.components(separatedBy: .newlines).joined()

        // Create JavaScript that loads the CSS
        let javaScript = """
           var element = document.createElement('style');
           element.innerHTML = '\(cssString)';
           document.head.appendChild(element);
        """
        
        // Create user script that inject the JavaScript after the HTML finishes loading
        let userScript = WKUserScript(source: javaScript,
                                      injectionTime: .atDocumentEnd,
                                      forMainFrameOnly: true)

        // Set user script to a configuration object and load it into the webView
        let userContentController = WKUserContentController()
        userContentController.addUserScript(userScript)
        let configuration = WKWebViewConfiguration()
        configuration.userContentController = userContentController
        let webView = WKWebView(frame: CGRect.zero, configuration: configuration)
        
        // Show web view on screen
        view.addSubview(webView)
        webView.layer.cornerRadius = 20.0
        webView.layer.masksToBounds = true
        webView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            webView.topAnchor.constraint(equalTo: view.layoutMarginsGuide.topAnchor, constant: 20.0),
            webView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 20.0),
            webView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -20.0),
            webView.bottomAnchor.constraint(equalTo: view.layoutMarginsGuide.bottomAnchor, constant: -20.0),
        ])
        
        // Load content to webView
        DispatchQueue.global().async {
            
            let link = "https://swiftsenpai.com/wp-content/uploads/2022/01/plainHtmlSample.html"
            guard
                let url = URL(string: link),
                let htmlContent = try? String(contentsOf: url, encoding: .utf8) else {
                assertionFailure("Fail to get agreement HTML")
                return
            }
            
            DispatchQueue.main.async {

                // Prepend `<meta>` viewport element to the HTML so that it will scale correctly in a mobile device
                let metaTag = "<meta name=\"viewport\" content=\"user-scalable=no, width=device-width\">"
                let html = "\(metaTag)\(htmlContent)"

                // Load HTML string to web view in main thread
                webView.loadHTMLString(html, baseURL: nil)
            }
        }
    }
}