Remove <<<--- add content here --->>> comments (#276)

These placeholders aren't useful. There is some content in all cases.

files/en-us/mdn/contribute/processes/matching_features_to_browser_version/index.html

@@ -126,14 +126,10 @@ <h3 id="EdgeHTML">EdgeHTML</h3>
 
 <p>Before Edge switched to Chromium, it used an engine developed by Microsoft called EdgeHTML. These resources will help you identify when features were added or removed in EdgeHTML-based versions of Edge—that is, Edge version 18 and earlier.</p>
 
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 <h3 id="Blink_Chromium">Blink (Chromium)</h3>
 
 <p>After Edge 18, Edge adopted the Blink engine, becoming based upon Chromium. The version number jumped to 79, to match the Chromium version number. While most features will be the same between all Chromium browsers at the same version number, there may be some exceptions. Here are some resources to help you identify those:</p>
 
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 <h2 id="Firefox">Firefox</h2>
 
 <p>There are several ways you can determine which browser release a feature arrived in. We'll look at a few of them, starting with the quickest and easiest way. Before we look at those, though, let's review some basic information you need before applying any of these techniques.</p>
@@ -351,16 +347,12 @@ <h3 id="Presto">Presto</h3>
 
 <p>Before Opera switched to Chromium, it used an engine developed by Opera called Presto. These resources will help you identify when features were added or removed in Presto-based versions of Opera—that is, Opera version 12.1 and earlier.</p>
 
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 <h3 id="Blink_Chromium_2">Blink (Chromium)</h3>
 
 <p>After Opera 12.10, Edge adopted the Blink engine, becoming based upon Chromium. The version number jumped to 15. The Blink engine version became Opera's version plus 13 -- that is, Opera 15 used Blink 28 (15+13=28) -- until Opera Desktop 69 and Opera Android 43. The browser files within the browser-compat-data project contain a mapping of Opera versions to Chromium/Blink versions; see <a href="https://github.com/mdn/browser-compat-data/blob/master/browsers/opera.json">browsers/opera.json</a> and <a href="https://github.com/mdn/browser-compat-data/blob/master/browsers/opera_android.json">browsers/opera_android.json</a>.</p>
 
 <p>While most features will be the same between all Chromium browsers, there may be some exceptions. Here are some resources to help you identify those:</p>
 
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 <h2 id="Safari">Safari</h2>
 
 <p>Fortunately, the WebKit source repository, found in SVN and available on the web using Trac, has branches for every WebKit version number, and tags for each Safari version. This makes it actually quite easy to narrow down when a given feature arrives. Find when the entry point for the feature was added, which is often in an IDL file. The WebKit version number at the time is in a <code>Source/WebCore/Configurations/Version.xcconfig</code> or <code>WebCore/Configurations/Version.xcconfig</code> file. The release branch may have already happened, so go through the branches or tags to verify that it was included.</p>
@@ -387,5 +379,3 @@ <h3 id="Useful_links">Useful links</h3>
 <div class="notecard warning">
 <p>Version numbers listed in the documentation at <a href="https://developer.apple.com/documentation/webkitjs">https://developer.apple.com/documentation/webkitjs</a> are inaccurate.  Please use other resources to determine the version numbers.</p>
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