Ask HN: How to cache webpages for offline viewing in WKWebView for iOS?

Greetings,

I am looking to cache certain relatively simple webpages for offline viewing using WKWebView in iOS. I am having trouble figuring out how to begin here. I saw this for UIWebView: https://github.com/buzzfeed/mattress.

Might anyone have any suggestions as to where I might begin on this? I know I can get the HTML for a page by making a javascript call from swift, but this of course won't give me the images.

Thank you very much for your time in advance.