Add dangling markup mitigation to the spec

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@@ -7079,21 +7079,36 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
   several wrappers to abstract base URLs and encodings. <ref>URL</ref></p>
 
   <p class="note">Most new APIs are to use <span>parse a URL</span>. Older APIs and HTML elements
-  might have reason to use <span data-x="encoding-parsing a URL">encoding-parse a URL</span>. When a
-  custom base URL is needed or no base URL is desired, the <span>URL parser</span> can of course be
-  used directly as well.</p>
+  might have reason to use <span data-x="encoding-parsing a URL">encoding-parse a URL</span>. If a
+  URL parsing was invoked as a result of HTML parsing (i.e. not from an API call), then
+  <span>parse a URL</span> should be used instead of calling <span>URL parser</span> directly.</p>
 
-  <p>To <dfn export>parse a URL</dfn>, given a string <var>url</var>, relative to a
-  <code>Document</code> object or <span>environment settings object</span> <var>environment</var>,
-  run these steps. They return failure or a <span>URL</span>.</p>
+  <p>To <dfn export>parse a URL</dfn>, given a <span>scalar value string</span> <var>url</var>,
+  relative to a <code>Document</code> object or <span>environment settings object</span>
+  <var>environment</var>,and an optional <span>encoding</span> <var>encoding</var> (default
+  <span>UTF-8</span>), run these steps. They return failure or a <span>URL</span>.</p>
 
   <ol>
    <li><p>Let <var>baseURL</var> be <var>environment</var>'s <span data-x="document base URL">base
    URL</span>, if <var>environment</var> is a <code>Document</code> object; otherwise
    <var>environment</var>'s <span>API base URL</span>.</p></li>
+
+   <li><p>Let <var>newline</var> and <var>lt</var> flag be false.</p></li>
+
+   <li><p>For each <var>byte</var> in <var>url</var>:</p>
+    <ol>
+     <li><p>If <var>byte</var> is an <span>ASCII tab or newline</span> character, then set the
+     <var>newline</var> flag to true.</p></li>
+
+     <li><p>Otherwise, if <var>byte</var> is a U+003C (&lt;) character, set the <var>lt</var> flag
+     to true.</p></li>
+    </ol>
+   </li>
+
+   <li><p>If both <var>newline</var> and <var>lt</var> flags are true, then return failure.</p></li>
 
    <li><p>Return the result of applying the <span>URL parser</span> to <var>url</var>, with
-   <var>baseURL</var>.</p></li>
+   <var>baseURL</var> and <var>encoding</var>.</p></li>
   </ol>
 
   <p id="resolve-a-url">To <dfn export data-x="encoding-parsing a URL">encoding-parse a URL</dfn>,
@@ -7114,12 +7129,12 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
    <code>Document</code></span>'s <span data-x="document's character encoding">character
    encoding</span>.</p></li>
 
-   <li><p>Let <var>baseURL</var> be <var>environment</var>'s <span data-x="document base URL">base
-   URL</span>, if <var>environment</var> is a <code>Document</code> object; otherwise
-   <var>environment</var>'s <span>API base URL</span>.</p></li>
+   <li><p>If <var>environment</var> is a <code>Document</code> object, then return the result of
+   <span data-x="parse a URL">parsing a URL</span> with <var>url</var> and <var>encoding</var>.</p>
+   </li>
 
-   <li><p>Return the result of applying the <span>URL parser</span> to <var>url</var>, with
-   <var>baseURL</var> and <var>encoding</var>.</p></li>
+   <li>Otherwise, return the result of applying the <span>URL parser</span> to <var>url</var>, with
+   <var>environment</var>'s <span>API base URL</span> and <var>encoding</var>.</p></li>
   </ol>
 
   <p>To <dfn data-x="encoding-parsing-and-serializing a URL">encoding-parse-and-serialize a
@@ -14962,7 +14977,7 @@ interface <dfn interface>HTMLBaseElement</dfn> : <span>HTMLElement</span> {
   <ol>
    <li><p>Let <var>document</var> be <var>element</var>'s <span>node document</span>.
 
-   <li><p>Let <var>urlRecord</var> be the result of <span data-x="URL parser">parsing</span> the
+   <li><p>Let <var>urlRecord</var> be the result of <span data-x="parse a URL">parsing</span> the
    value of <var>element</var>'s <code data-x="attr-base-href">href</code> content attribute with
    <var>document</var>'s <span>fallback base URL</span>, and <var>document</var>'s <span
    data-x="document's character encoding">character encoding</span>. (Thus, the <code>base</code>
@@ -34351,7 +34366,7 @@ interface <dfn interface>HTMLObjectElement</dfn> : <span>HTMLElement</span> {
         <!-- if we get to this point we know we can successfully parsed the URL, since this
         algorithm is only used after fetching the resource in the steps above -->
 
-        <p>If applying the <span>URL parser</span> algorithm to the <span>URL</span> of the
+        <p>If applying the <span>parse a URL</span> algorithm to the <span>URL</span> of the
         specified resource (after any redirects) results in a <span>URL record</span> whose <span
         data-x="concept-url-path">path</span> component matches a pattern that a <span>plugin</span>
         supports, then let <var>resource type</var> be the type that that plugin can
@@ -103841,7 +103856,7 @@ new PaymentRequest(&hellip;); // Allowed to use
 
    <dd>
     <p>A <span>URL</span> used by APIs called by scripts that use this <span>environment settings
-    object</span> to <span data-x="parse a URL">parse URLs</span>.</p>
+    object</span> to <span data-x="URL parser">parse URLs</span>.</p>
    </dd>
 
    <dt>An <dfn data-x="concept-settings-object-origin" export for="environment
@@ -112360,7 +112375,7 @@ interface <dfn interface>Navigator</dfn> {
      <li><p>Replace the first instance of "<code data-x="">%s</code>" in <var>handlerURLString</var>
      with <var>encodedURL</var>.</p></li>
 
-     <li><p>Let <var>resultURL</var> be the result of <span data-x="URL parser">parsing</span>
+     <li><p>Let <var>resultURL</var> be the result of <span data-x="parse a URL">parsing</span>
      <var>handlerURLString</var>.</p></li>
 
      <li><p><span>Navigate</span><!--DONAV user--> an appropriate <span>navigable</span> to