[lighthouse] add "mixed content" reference

src/content/en/tools/lighthouse/_toc.yaml

@@ -116,6 +116,8 @@ toc:
   path: /web/tools/lighthouse/audits/http-200-when-offline
 - title: Serve Images As WebP
   path: /web/tools/lighthouse/audits/webp
+- title: Some Insecure Resources Can Be Upgraded To HTTPS
+  path: /web/tools/lighthouse/audits/mixed-content
 - title: Time to Interactive
   path: /web/tools/lighthouse/audits/time-to-interactive
 - title: Unoptimized Images

src/content/en/tools/lighthouse/audits/mixed-content.md

@@ -0,0 +1,66 @@
+project_path: /web/tools/_project.yaml
+book_path: /web/tools/_book.yaml
+description: Reference documentation for the "Some Insecure Resources Can Be Upgraded To HTTPS" Lighthouse audit.
+
+{# wf_updated_on: 2018-02-06 #}
+{# wf_published_on: 2018-02-06 #}
+{# wf_blink_components: Platform>DevTools #}
+
+# Some Insecure Resources Can Be Upgraded To HTTPS  {: .page-title }
+
+## Overview {: #overview }
+
+When a secure (HTTPS) site requests an insecure (HTTP) resource, that is called a [mixed
+content][MC] error. Some browsers block insecure resource requests by default. If your page
+depends on these insecure resources, then your page might not work properly when they get blocked.
+
+[MC]: /web/fundamentals/security/prevent-mixed-content/what-is-mixed-content
+
+<figure>
+  <img src="images/insecure-content-blocked.png"
+       alt="Google Chrome (version 66) blocking insecure content."/>
+  <figcaption>
+    <b>Figure X</b>. Google Chrome (version 66) blocking insecure content
+  </figcaption>
+</figure>
+
+Also, browsers may warn users that your page is not fully secure.
+
+<figure>
+  <img src="images/not-fully-secure.png"
+       alt="Google Chrome (version 66) reporting that page is not fully secure."/>
+  <figcaption>
+    <b>Figure X</b>. Google Chrome (version 66) reporting that a page is not fully secure
+  </figcaption>
+</figure>
+
+## Recommendations {: #recommendations }
+
+For each URL that Lighthouse lists, modify your code to request that URL over HTTPS:
+
+    https://example.com
+
+Rather than HTTP:
+
+    http://example.com
+
+Even if you are not ready to fully upgrade your site to HTTPS yet, upgarding some URLS to
+HTTPS can increase the security of your users and make it easier for you to avoid mixed
+content warnings when you do upgrade to HTTPS in the future. See [Uses HTTPS][https] for
+recommendations on how to upgrade your site to HTTPS.
+
+Insecure resources which are not upgradeable to HTTPS may require other changes. Contact the
+providers of these resources to explore your options.
+
+[https]: /web/tools/lighthouse/audits/https#recommendations
+
+## More information {: #more-info }
+
+Lighthouse loads the page once to record the URL of each resource requested. Then, Lighthouse
+loads the page again, but this time intercepts each request and tries to upgrade the URL to
+use HTTPS. If the upgraded requests succeed, Lighthouse reports that request as upgradeable in
+the audit results.
+
+[Audit source][src]{:.external}
+
+[src]: https://github.com/GoogleChrome/lighthouse/blob/master/lighthouse-core/audits/mixed-content.js