Add og:image:alt for accessibility. (#3066)

* dist/index: Add og:image:alt for accessibility.

* src/index, docs/(html|extend): Add og:image:alt everywhere.

dist/index.html

@@ -12,6 +12,7 @@
   <meta property="og:type" content="">
   <meta property="og:url" content="">
   <meta property="og:image" content="">
+  <meta property="og:image:alt" content="">
 
   <link rel="icon" href="/favicon.ico" sizes="any">
   <link rel="icon" href="/icon.svg" type="image/svg+xml">

docs/extend.md

@@ -211,6 +211,8 @@ Facebook).
 <meta property="og:type" content="website">
 <meta property="og:title" content="">
 <meta property="og:image" content="https://www.example.com/path/to/image.jpg">
+<!-- Empty for decorative images. -->
+<meta property="og:image:alt" content="Example image depicting...">
 <meta property="og:description" content="">
 <meta property="og:site_name" content="">
 <meta property="article:author" content="">

docs/html.md

@@ -80,6 +80,7 @@ website, article).
 <meta property="og:type" content="">
 <meta property="og:url" content="">
 <meta property="og:image" content="">
+<meta property="og:image:alt" content="">
 ```
 
 In addition to these four attributes there are many more attributes you can use
@@ -96,6 +97,8 @@ describe the site in more detail.
 <meta property="og:type" content="website">
 <meta property="og:description" content="The web’s most popular front-end template which helps you build fast, robust, and adaptable web apps or sites.">
 <meta property="og:image" content="https://html5boilerplate.com/icon.png">
+<!-- Empty for decorative images. -->
+<meta property="og:image:alt" content="">
 ```
 
 ### Web App Manifest

src/index.html

@@ -12,6 +12,7 @@
   <meta property="og:type" content="">
   <meta property="og:url" content="">
   <meta property="og:image" content="">
+  <meta property="og:image:alt" content="">
 
   <link rel="icon" href="/favicon.ico" sizes="any">
   <link rel="icon" href="/icon.svg" type="image/svg+xml">