add JSON-LD conversion

fix #29

Note this just uses the changes @lanthaler said were needed back in
2012, so it should be checked before merging.

index.html

@@ -1238,6 +1238,192 @@ <h3>Associating names with items</h3>
 <section id="converting-html-to-other-formats">
 <h2>Converting Microdata to other formats</h2>
 
+<section id="jsonLD">
+<h3>JSON-LD</h3>
+
+  <p>Given a list of nodes <var>nodes</var> in a <code>Document</code>, a user agent must
+  run the following algorithm to extract the Microdata from those nodes into a JSON-LD:</p>
+
+  <ol>
+
+   <li><p>Let <var>result</var> be an empty object.</p></li>
+
+   <li><p>Let <var>items</var> be an empty array.</p></li>
+
+   <li><p>For each <var>node</var> in <var>nodes</var>, check if the element is a
+   <a>top-level Microdata item</a>, and if it is then <a>get the object</a> for that element and add it to <var>items</var>.</p></li>
+
+   <li><p>Add an entry to <var>result</var> called "<code>items</code>" whose
+   value is the array <var>items</var>.</p></li>
+
+   <li><p>Add an entry to <var>result</var> called "<code>@context</code>" whose value is the following object:</p>
+     <pre class="html">{ "@vocab" : "" }</pre></li>
+
+   <li><p>Return the result of serializing <var>result</var> to JSON. [[JSON]]</p></li>
+
+  </ol>
+
+  <p class="note">This algorithm returns an object with a single property that is an array, instead
+  of just returning an array, so that it is possible to extend the algorithm in the future if
+  necessary.</p>
+
+  <p>When the user agent is to <dfn>get the object</dfn> for an item <var>item</var>,
+  potentially together with a list of elements <var>memory</var>, it must run the following
+  substeps:</p>
+
+  <ol>
+
+   <li><p>Let <var>result</var> be an empty object.</p></li>
+
+   <li><p>If no <var>memory</var> was passed to the algorithm, let <var>memory</var> be an empty list.</p></li>
+
+   <li><p>Add <var>item</var> to <var>memory</var>.</p></li>
+
+   <li><p>If the <var>item</var> has any <a>item types</a>, add an entry to <var>result</var>
+   called "<code>@type</code>" whose value is an array listing the
+   <a>item types</a> of <var>item</var>, in the order they were specified on the
+   <code><a>itemtype</a></code> attribute.</p>
+
+   </li><li><p>If the <var>item</var> has a <a>global identifier</a>, add an entry to
+   <var>result</var> called "<code>@id</code>" whose value is the <a>global
+   identifier</a> of <var>item</var>.</p></li>
+
+   <li><p>Let <var>properties</var> be an empty object.</p></li>
+
+   <li>
+
+    <p>For each element <var>element</var> that has one or more <a>property names</a>
+    and is one of <a data-lt="item properties">the properties of the item</a>
+    <var>item</var>, in the order those elements are given by the algorithm that returns
+    <a data-lt="item properties">the properties of an item</a>, run the following substeps:</p>
+
+    <ol>
+
+     <li><p>Let <var>value</var> be the
+     <a>property value</a> of <var>element</var>.</p></li>
+
+     <li><p>If <var>value</var> is an <a data-lt="concept item">item</a>, then:
+     If <var>value</var> is in <var>memory</var>, then let <var>value</var> be
+     the string "<code>ERROR</code>". Otherwise, <a>get the object</a> for
+     <var>value</var>, passing a copy of <var>memory</var>, and then replace <var>value</var>
+     with the object returned from those steps.</p></li>
+
+     <li>
+
+      <p>For each name <var>name</var> in <var>element</var>'s
+      <a>property names</a>, run the following substeps:</p>
+
+      <ol>
+
+       <li><p>If there is no entry named <var>name</var> in <var>result</var>,
+       then add an entry named <var>name</var> to <var>result</var> whose
+       value is an empty array.</p></li>
+
+       <li><p>Append <var>value</var> to the entry named <var>name</var> in <var>result</var>.</p></li>
+
+      </ol>
+
+     </li>
+
+    </ol>
+
+   </li>
+
+   <li><p>Return <var>result</var>.</p></li>
+
+  </ol>
+
+  <div class="example">
+
+   <p>For example, take this markup:</p>
+
+   <pre class="html">&lt;!DOCTYPE HTML&gt;
+&lt;title&gt;My Blog&lt;/title&gt;
+&lt;article itemscope itemtype="https://schema.org/BlogPosting"&gt;
+ &lt;header&gt;
+  &lt;h1 itemprop="headline"&gt;Progress report&lt;/h1&gt;
+  &lt;p&gt;&lt;time itemprop="datePublished" datetime="2013-08-29"&gt;today&lt;/time&gt;&lt;/p&gt;
+  &lt;link itemprop="url" href="?comments=0"&gt;
+ &lt;/header&gt;
+ &lt;p&gt;All in all, he's doing well with his swim lessons. The biggest thing was he had trouble
+ putting his head in, but we got it down.&lt;/p&gt;
+ &lt;section&gt;
+  &lt;h1&gt;Comments&lt;/h1&gt;
+  &lt;article itemprop="comment" itemscope itemtype="https://schema.org/Comment" id="c1"&gt;
+   &lt;link itemprop="url" href="#c1"&gt;
+   &lt;footer&gt;
+    &lt;p&gt;Posted by: &lt;span itemprop="creator" itemscope itemtype="https://schema.org/Person"&gt;
+     &lt;span itemprop="name"&gt;Greg&lt;/span&gt;
+    &lt;/span&gt;&lt;/p&gt;
+    &lt;p&gt;&lt;time itemprop="dateCreated" datetime="2013-08-29"&gt;15 minutes ago&lt;/time&gt;&lt;/p&gt;
+   &lt;/footer&gt;
+   &lt;p&gt;Ha!&lt;/p&gt;
+  &lt;/article&gt;
+  &lt;article itemprop="comment" itemscope itemtype="https://schema.org/Comment" id="c2"&gt;
+   &lt;link itemprop="url" href="#c2"&gt;
+   &lt;footer&gt;
+    &lt;p&gt;Posted by: &lt;span itemprop="creator" itemscope itemtype="https://schema.org/Person"&gt;
+     &lt;span itemprop="name"&gt;Charlotte&lt;/span&gt;
+    &lt;/span&gt;&lt;/p&gt;
+    &lt;p&gt;&lt;time itemprop="dateCreated" datetime="2013-08-29"&gt;5 minutes ago&lt;/time&gt;&lt;/p&gt;
+   &lt;/footer&gt;
+   &lt;p&gt;When you say "we got it down"...&lt;/p&gt;
+  &lt;/article&gt;
+ &lt;/section&gt;
+&lt;/article&gt;</pre>
+
+   <p>It would be turned into the following JSON by the algorithm above (supposing that the page's
+   URL was <code>http://blog.example.com/progress-report</code>):</p>
+
+   <pre>{
+  "items": [
+    {
+      "type": [ "https://schema.org/BlogPosting" ],
+      "properties": {
+        "headline": [ "Progress report" ],
+        "datePublished": [ "2013-08-29" ],
+        "url": [ "http://blog.example.com/progress-report?comments=0" ],
+        "comment": [
+          {
+            "type": [ "https://schema.org/Comment" ],
+            "properties": {
+              "url": [ "http://blog.example.com/progress-report#c1" ],
+              "creator": [
+                {
+                  "type": [ "https://schema.org/Person" ],
+                  "properties": {
+                    "name": [ "Greg" ]
+                  }
+                }
+              ],
+              "dateCreated": [ "2013-08-29" ]
+            }
+          },
+          {
+            "type": [ "https://schema.org/Comment" ],
+            "properties": {
+              "url": [ "http://blog.example.com/progress-report#c2" ],
+              "creator": [
+                {
+                  "type": [ "https://schema.org/Person" ],
+                  "properties": {
+                    "name": [ "Charlotte" ]
+                  }
+                }
+              ],
+              "dateCreated": [ "2013-08-29" ]
+            }
+          }
+        ]
+      }
+    }
+  ]
+}</pre>
+
+  </div>
+</section>
+
+
 <section id="json">
 <h3>JSON</h3>
 
@@ -1653,6 +1839,7 @@ <h2>Changes</h2>
     and the <a href="https://www.w3.org/TR/2017/WD-microdata-20170626/">second Working Draft</a>:</p>
 
   <ul>
+    <li>Add a section describing how to convert <a href="#jsonLD">microdata to JSON-LD</a>.</li>
     <li><a>itemid</a> may be specified on an element with an <a>itemscope</a> attribute, not just a <a>typed item</a></li>
   </ul>
 
@@ -1699,14 +1886,17 @@ <h2>Acknowledgements</h2>
 Christine Runnegar,
 Gregg Kellogg,
 Ivan Herman,
+Jeni Tennison,
 Jens Oliver Meiert,
 Léonie Watson,
 Manu Sporny,
-Marcos Caceres,
+Marcos Cáceres,
+Markus Lanthaler,
 Nick Doty,
 Philippe Le Hégaret,
 Ralph Swick,
 Robin Berjon,
+Shane McCarron,
 Tab Atkins,
 Tavis Tucker,
 Tobie Langel,