Webview in modal view.

The benefit to try things and do things little different make you wonder if its possible to put something else in a accessory view. And the only thing I did was to add 2 handlers and created
some new arguments. That is the benefit to have handlers that are like building blocks.

This example use webview…

Ps. Okey admit it was not so simple, I need a way to abort the running modal loop.

use framework "Foundation"
use framework "AppKit"
use scripting additions

property cancelButton : "Cancel"
property returnCode : missing value

on run
	set webView to showAlert given arguments:{title:"WebView", webURL:"https://www.apple.com", message:"My message...", info:"My info...", buttons:{"Cancel", "OK"}}
end run

on showAlert given arguments:arguments
		if my NSThread's isMainThread() as boolean then
			-- if we run in main thread set the arguments to dictionary.
			set arguments to current application's NSDictionary's dictionaryWithDictionary:arguments
			my performAlert:arguments
			my performSelectorOnMainThread:"performAlert:" withObject:arguments waitUntilDone:true
		end if
		if button returned of returnCode is cancelButton then error number -128
		return returnCode
	on error error_message number error_number
		tell me to activate
		display alert "" & error_number & "" message error_message
		return returnCode
	end try
end showAlert

on performAlert:arguments
	set theView to createWebView(arguments's webURL, 400, 200)
	set theAlert to createAlert(arguments's title, arguments's message, arguments's info, arguments's buttons, theView)
	set button to theAlert's runModal()
	set buttonsArray to (buttons of arguments)'s reverseObjectEnumerator()'s allObjects()
	set button to item (button mod 1000 + 1) of buttonsArray
	set my returnCode to {button returned:button as text}
end performAlert:

on createAlert(title, message, info, buttons, theView)
	set theAlert to current application's NSAlert's alloc()'s init()
	theAlert's |window|'s setTitle:title
	theAlert's setMessageText:message
	theAlert's setInformativeText:info
	repeat with anItem in reverse of (buttons as list)
		(theAlert's addButtonWithTitle:anItem)
	end repeat
	-- Set the accessoryView to the alert.
	theAlert's setAccessoryView:theView
	return theAlert
end createAlert

on createWebView(theURLString, width, height)
	set theConfiguration to current application's WKWebViewConfiguration's alloc()'s init()
	set theFetch to my fetchJSSourceString(theURLString)
	set theUserScript to current application's WKUserScript's alloc()'s initWithSource:theFetch injectionTime:(current application's WKUserScriptInjectionTimeAtDocumentEnd) forMainFrameOnly:true
	set theUserContentController to current application's WKUserContentController's alloc()'s init()
	theUserContentController's addUserScript:theUserScript
	theConfiguration's setUserContentController:theUserContentController
	set webViewSize to current application's NSMakeRect(0, 0, width, height)
	set webView to current application's WKWebView's alloc()'s initWithFrame:webViewSize configuration:theConfiguration
	webView's setNavigationDelegate:me
	webView's setUIDelegate:me
	webView's setTranslatesAutoresizingMaskIntoConstraints:true
	set theURL to current application's |NSURL|'s URLWithString:theURLString
	set theRequest to current application's NSURLRequest's requestWithURL:theURL
	webView's loadRequest:theRequest
	return webView
end createWebView

on fetchJSSourceString(theURLString)
	set theURL to current application's |NSURL|'s URLWithString:theURLString
	set theSource to current application's NSString's stringWithContentsOfURL:theURL encoding:(current application's NSUTF8StringEncoding) |error|:(missing value)
	return theSource
end fetchJSSourceString