Issue 6441 - Avoid unnecessary shadow root

include.preload.js

Index: include.preload.js
===================================================================
--- a/include.preload.js
+++ b/include.preload.js
@@ -370,17 +370,21 @@
     if (/\.(?:google|blogger)\.com$/.test(document.domain))
       return null;
 
     // Finally since some users have both AdBlock and Adblock Plus installed we
     // have to consider how the two extensions interact. For example we want to
     // avoid creating the shadowRoot twice.
     let shadow = document.documentElement.shadowRoot ||
                  document.documentElement.createShadowRoot();
-    shadow.appendChild(document.createElement("shadow"));
+
+    // Firefox does not support the <shadow> tag (#6441).
+    let contentTagName = typeof HTMLShadowElement != "undefined" ?
+                         "shadow" : "content";

[Sebastian Noack, 2018/03/08 18:34:26]

I wonder whether we should just use <content> no matter what. Since we only
create the shadow root if there is no shadow root yet (which is only possible if
it was created by another extension, anyway, due to the timing), <content> and
<shadow> have essentially the same effect.

[Manish Jethani, 2018/03/08 19:44:54]

Are you sure that Chrome 49 supports <content>? I don't know, but I could check.
Yes it would be nice to use <content> always.

[Manish Jethani, 2018/03/08 19:53:57]

I checked the Chromium commit log and entries mentioning <content> go way back
several years. I'm pretty sure it's supported in 49.

Done.

[Sebastian Noack, 2018/03/08 20:36:16]

Yes, I'm pretty sure <content> already existed when we initially started using
Shadow DOM. The reason we didn't use it back then, was because we initially
created the shadow root lazily, and wanted to avoid breaking websites that
created their own shadow root for the document element.

+    shadow.appendChild(document.createElement(contentTagName));
 
     return shadow;
   },
 
   addSelectorsInline(selectors, groupName)
   {
     let style = this.styles.get(groupName);