Add site and obtain a site

It's useful for whatwg/fetch#943 to be able to distinguish between a site and agent cluster key as #4734 changes how agent clusters can be keyed.

source

@@ -91747,11 +91747,13 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
   <p>A <dfn data-export="">scheme-and-registrable-domain</dfn> is a <span>tuple</span> of a <span
   data-x="concept-url-scheme">scheme</span> and a <span data-x="concept-domain">domain</span>.</p>
 
-  <p>An <dfn data-export="">agent cluster key</dfn> is an <span>origin</span> or a
+  <p>A <dfn data-export="">site</dfn> is an <span data-x="concept-origin-opaque">opaque
+  origin</span>, a <span data-x="concept-origin-tuple">tuple origin</span> whose <span
+  data-x="concept-origin-host">host</span>'s <span>registrable domain</span> is null, or a
   <span>scheme-and-registrable-domain</span>.</p>
 
-  <p>To <dfn data-x="obtain-agent-cluster-key">obtain an agent cluster key</dfn>, given an origin
-  <var>origin</var>, run these steps:</p>
+  <p>To <dfn data-export="">obtain a site</dfn>, given an origin <var>origin</var>, run these
+  steps:</p>
 
   <ol>
    <li><p>If <var>origin</var> is an <span data-x="concept-origin-opaque">opaque origin</span>, then
@@ -91765,6 +91767,16 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
    domain</span>).</p></li>
   </ol>
 
+  <p>An <dfn>agent cluster key</dfn> is a <span>site</span>.</p>
+
+  <p class="XXX"><a href="https://github.com/whatwg/html/pull/4734">whatwg/html #4734</a> is
+  expected to widen <span>agent cluster key</span> (but not <span>site</span>) to encompass all
+  <span data-x="concept-origin-tuple">tuple origins</span>.</p>
+
+  <p>To <dfn data-x="obtain-agent-cluster-key">obtain an agent cluster key</dfn>, given an origin
+  <var>origin</var>, return the result of <span data-x="obtain a site">obtaining a site</span> with
+  <var>origin</var>.</p>
+
   <p>To <dfn data-x="obtain-similar-origin-window-agent">obtain a similar-origin window agent</dfn>,
   given an <span>origin</span> <var>origin</var> and <span>browsing context group</span>
   <var>group</var>, run these steps:</p>