Legacy warning

Author: jakearchibald

fetch.bs

@@ -6404,9 +6404,13 @@ constructor steps are:
 
    <li><p>Empty <a>this</a>'s <a for=Request>headers</a>'s <a for=Headers>header list</a>.
 
-   <li><p>If <a>this</a>'s <a for=Request>request</a>'s <a for=request>mode</a> is
-   "<code>no-cors</code>" and <a>this</a>'s <a for=Request>request</a>'s <a for=request>method</a>
-   is not a <a>CORS-safelisted method</a>, then <a>throw</a> a {{TypeError}}.
+   <li>
+    <p>If <a>this</a>'s <a for=Request>request</a>'s <a for=request>mode</a> is
+    "<code>no-cors</code>" and <a>this</a>'s <a for=Request>request</a>'s <a for=request>method</a>
+    is not a <a>CORS-safelisted method</a>, then <a>throw</a> a {{TypeError}}.
+
+    <p class=note>If <var>init</var> <a for=map>is empty</a> and <var>input</var> is a {{Request}}, the <a for=request>mode</a> and <a for=request>method</a> will be copied without the above check. This is considered legacy. Modern APIs should instead use a request with a <a for=request>mode</a> of "<code>cors</code>" if they want to use a <a for=request>method</a> that is not a <a>CORS-safelisted method</a>.
+   </li>
 
    <li><p>If <var>headers</var> is a <code>Headers</code> object, then <a for=list>for each</a>
    <var>header</var> in its <a for=Headers>header list</a>, <a for=Headers>append</a>