I’d assumed that @Sthuv was trying something different as I covered that very point in the first reply 
If it is just an iCloud file, Scriptable can getthe file content and load the HTML into the WebView : use loadHTML() or loadFile() (for info, Simon published an example with loadFile() a few years back).
If it is a file hosted on a web server, then you would pass the URL to the WebView to load: use loadURL().
These functions are covered in the WebView documentation