Showing how metatags are being used in modern webpages with a browser and crawlers. (omitting common tags, e.g. title, meta-charset)

mdn.html

<!-- https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel -->
<meta name="robots" content="index, follow">
<meta name="description" content="The rel attribute defines the relationship between a linked resource and the current document. Valid on &lt;link&gt;, &lt;a&gt;, &lt;area&gt;, and &lt;form&gt;, the supported values depend on the element on which the attribute is found."/>
<meta property="og:url" content="https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel"/>
<meta property="og:title" content="HTML attribute: rel - HTML: HyperText Markup Language | MDN"/>
<meta property="og:type" content="website"/>
<meta property="og:locale" content="en_US"/>
<meta property="og:description" content="The rel attribute defines the relationship between a linked resource and the current document. Valid on &lt;link&gt;, &lt;a&gt;, &lt;area&gt;, and &lt;form&gt;, the supported values depend on the element on which the attribute is found."/>
<meta property="og:image" content="https://developer.mozilla.org/mdn-social-share.cd6c4a5a.png"/>
<meta property="og:image:type" content="image/png"/>
<meta property="og:image:height" content="1080"/>
<meta property="og:image:width" content="1920"/>
<meta property="og:image:alt" content="The MDN Web Docs logo, featuring a blue accent color, displayed on a solid black background."/>
<meta property="og:site_name" content="MDN Web Docs"/>
<meta name="twitter:card" content="summary_large_image"/>
<meta name="twitter:creator" content="MozDevNet"/>
<meta name="theme-color" content="#ffffff"/>
<link rel="manifest" href="/manifest.56b1cedc.json"/>
<link rel="search" type="application/opensearchdescription+xml" href="/opensearch.xml" title="MDN Web Docs"/>
<link rel="icon" href="/favicon-48x48.cbbd161b.png"/>
<link rel="apple-touch-icon" href="/apple-touch-icon.6803c6f0.png"/>
<link rel="alternate" title="Tipos de enlaces" href="https://developer.mozilla.org/es/docs/Web/HTML/Attributes/rel" hreflang="es"/>
<link rel="alternate" title="Attribut HTML : rel" href="https://developer.mozilla.org/fr/docs/Web/HTML/Attributes/rel" hreflang="fr"/>
<link rel="alternate" title="HTML 属性: rel" href="https://developer.mozilla.org/ja/docs/Web/HTML/Attributes/rel" hreflang="ja"/>
<link rel="alternate" title="Link types" href="https://developer.mozilla.org/ko/docs/Web/HTML/Attributes/rel" hreflang="ko"/>
<link rel="alternate" title="HTML 属性：rel" href="https://developer.mozilla.org/zh-CN/docs/Web/HTML/Attributes/rel" hreflang="zh"/>
<link rel="alternate" title="HTML attribute: rel" href="https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel" hreflang="en"/>
<link rel="alternate" type="application/rss+xml" title="MDN Blog RSS Feed" href="https://developer.mozilla.org/en-US/blog/rss.xml" hreflang="en"/>
<link rel="canonical" href="https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel"/>

youtube.html

<!-- https://www.youtube.com -->
<link rel="shortcut icon" href="https://www.youtube.com/s/desktop/9cc6dbeb/img/favicon.ico" type="image/x-icon">
<link rel="icon" href="https://www.youtube.com/s/desktop/9cc6dbeb/img/favicon_32x32.png" sizes="32x32">
<link rel="icon" href="https://www.youtube.com/s/desktop/9cc6dbeb/img/favicon_48x48.png" sizes="48x48">
<link rel="icon" href="https://www.youtube.com/s/desktop/9cc6dbeb/img/favicon_96x96.png" sizes="96x96">
<link rel="icon" href="https://www.youtube.com/s/desktop/9cc6dbeb/img/favicon_144x144.png" sizes="144x144">
<meta property="og:image" content="https://www.youtube.com/img/desktop/yt_1200.png">
<meta property="fb:app_id" content="87741124305">
<link rel="canonical" href="https://www.youtube.com/">
<link rel="alternate" media="handheld" href="https://m.youtube.com/">
<link rel="alternate" media="only screen and (max-width: 640px)" href="https://m.youtube.com/">
<link rel="alternate" href="android-app://com.google.android.youtube/http/www.youtube.com/">
<link rel="alternate" href="ios-app://544007664/vnd.youtube/www.youtube.com/">