JXA (JavaScript for Automation) WebView applet template.

JXA-WebView.js

ObjC.import('Cocoa')
ObjC.import('WebKit')

MyWindow = WebViewWindow('http://www.youtube.com/')
MyWindow.makeKeyAndOrderFront(null)

//=====================================================================

function Window(x, y, width, height) {
  let r = $.NSMakeRect(x, y, width, height)
  let s = $.NSTitledWindowMask
      s|= $.NSClosableWindowMask
      s|= $.NSMiniaturizableWindowMask
  let b = $.NSBackingStoreBuffered
  let d = false
  let w = $.NSWindow.alloc.initWithContentRectStyleMaskBackingDefer(r, s, b, d)
  w.releasedWhenClosed = false
  if (x==-1 && y==-1) w.center
  return w
}

function WindowDelegate() {
  if (!$.WindowDelegate) ObjC.registerSubclass({
    name:'WindowDelegate',
    protocols: ['NSWindowDelegate'],
    methods: {
      'windowWillClose:' (notification) {
        return $.NSApplication.sharedApplication.terminate(null)
      },
    },
  })
  return $.WindowDelegate.alloc.init
}

function WebView(url) {
  let z = $.NSZeroRect
  let c = $.WKWebViewConfiguration.alloc.init
  let w = $.WKWebView.alloc.initWithFrameConfiguration(z, c)
  let u = $.NSURL.URLWithString(url)
  let r = $.NSURLRequest.requestWithURL(u)
  w.loadRequest(r)
  return w
}

function WebViewWindow(url, x=-1, y=-1, width=1024, height=576) {
  let web = WebView(url)
  web.frame = $.NSMakeRect(0, 0, width, height)
  web.autoresizingMask = $.NSViewWidthSizable | $.NSViewHeightSizable
  let win = Window(x, y, width, height)
  win.styleMask |= $.NSResizableWindowMask
  win.delegate = WindowDelegate()
  win.contentView.addSubview(web)
  return win
}

This is brilliant. You've saved me hours of trial and error. Thank you for sharing!