How to block external resources to load on a WKWebView?

In case anybody else is interested in an offline-only WKWebView: The following approach is a modification of @dequin's answer. It uses content blocking rules to block all requests to remote resources (URLs that don't start with file://):

import Cocoa
import WebKit

// Block all URLs except those starting with "file://"
let blockRules = """
[
    {
        "trigger": {
            "url-filter": ".*"
        },
        "action": {
            "type": "block"
        }
    },
    {
        "trigger": {
            "url-filter": "file://.*"
        },
        "action": {
            "type": "ignore-previous-rules"
        }
    }
]
"""

/// `WKWebView` which only allows the loading of local resources
class OfflineWebView: WKWebView {
    override init(frame: CGRect, configuration: WKWebViewConfiguration) {
        WKContentRuleListStore.default().compileContentRuleList(
            forIdentifier: "ContentBlockingRules",
            encodedContentRuleList: blockRules
        ) { contentRuleList, error in
            if let error = error {
                // Handle error
            } else if let contentRuleList = contentRuleList {
                configuration.userContentController.add(contentRuleList)
            } else {
                // Handle error
            }
        }

        super.init(frame: frame, configuration: configuration)
    }
}

Since iOS 11 you can use WKContentRuleList

First, create a Content Rule or a list. Each rule is comprised of a trigger and an action. See Apple's Documentation on Content Rule creation

This is a creation example, blocks all image and Style Sheet content, but allows those ended on jpeg by way of ignoring the previous rules:

     let blockRules = """
         [{
             "trigger": {
                 "url-filter": ".*",
                 "resource-type": ["image"]
             },
             "action": {
                 "type": "block"
             }
         },
         {
             "trigger": {
                 "url-filter": ".*",
                 "resource-type": ["style-sheet"]
             },
             "action": {
                 "type": "block"
             }
         },
         {
             "trigger": {
                 "url-filter": ".*.jpeg"
             },
             "action": {
                 "type": "ignore-previous-rules"
             }
         }]
      """        

Having your list of rules, you can add them to the ContentRuleListStore

    import WebKit
    @IBOutlet weak var wkWebView: WKWebView!

    let request = URLRequest(url: URL(string: "https://yourSite.com/")!)

    WKContentRuleListStore.default().compileContentRuleList(
        forIdentifier: "ContentBlockingRules",
        encodedContentRuleList: blockRules) { (contentRuleList, error) in

            if let error = error {
                return
            }

            let configuration = self.webView.configuration
            configuration.userContentController.add(contentRuleList!)

            self.wkWwebView.load(self.request)
    }

If later you want to remove all your rules, call:

    self.wkWebView.configuration.userContentController.removeAllContentRuleLists()
    self.wkWebView.load(self.request)

Here is the 2017 WWDC video

Best of lucks!

I've created a sample project on Github WKContentRuleExample