Draft integration with Trusted Types, take 2.

See w3c/trusted-types#418 and whatwg#789.
Supercedes PR whatwg#809.

dom.bs

@@ -6133,8 +6133,8 @@ interface Element : Node {
   sequence<DOMString> getAttributeNames();
   DOMString? getAttribute(DOMString qualifiedName);
   DOMString? getAttributeNS(DOMString? namespace, DOMString localName);
-  [CEReactions] undefined setAttribute(DOMString qualifiedName, DOMString value);
-  [CEReactions] undefined setAttributeNS(DOMString? namespace, DOMString qualifiedName, DOMString value);
+  [CEReactions] undefined setAttribute(DOMString qualifiedName, (TrustedType or DOMString) value);
+  [CEReactions] undefined setAttributeNS(DOMString? namespace, DOMString qualifiedName, (TrustedType or DOMString) value);
   [CEReactions] undefined removeAttribute(DOMString qualifiedName);
   [CEReactions] undefined removeAttributeNS(DOMString? namespace, DOMString localName);
   [CEReactions] boolean toggleAttribute(DOMString qualifiedName, optional boolean force);
@@ -6456,6 +6456,10 @@ given a <var>document</var>, <var>localName</var>, <var>namespace</var>, and opt
 <a>attribute</a> <var>attribute</var> to <var>value</var>, run these steps:
 
 <ol>
+<li><p>Set <var>value</var> to the result of calling <a>Get Trusted Types-compliant attribute
+ value</a> for <var>attribute</var>, with <var>attribute</var>'s <a for=Attr>element</a> and
+ <var>value</var>. [[!TRUSTED-TYPES]]
+
  <li><p>Let <var>oldValue</var> be <var>attribute</var>'s <a for=Attr>value</a>.</p></li>
 
  <li><p>Set <var>attribute</var>'s <a for=Attr>value</a> to <var>value</var>.
@@ -6469,6 +6473,11 @@ given a <var>document</var>, <var>localName</var>, <var>namespace</var>, and opt
 steps:
 
 <ol>
+<li><p>Set <var>attribute</var>'s
+ <a for=Attr>value</a> to the result of calling <a>Get Trusted Types-compliant attribute value</a>
+ for <var>attribute</var>, with <var>element</var> and <var>attribute</var>'s <a for=Attr>value</a>.
+ [[!TRUSTED-TYPES]]
+
  <li><p><a for=list>Append</a> <var>attribute</var> to <var>element</var>'s
  <a for=Element>attribute list</a>.
 
@@ -6497,6 +6506,11 @@ steps:
 <a>attribute</a> <var>oldAttr</var> with an <a>attribute</a> <var>newAttr</var>, run these steps:
 
 <ol>
+ <li><p>Set <var>newAttr</var>'s
+ <a for=Attr>value</a> to the result of calling <a>Get Trusted Types-compliant attribute value</a>
+ for <var>newAttr</var>, with <var>oldAttr</var>'s <a for=Attr>element</a> and <var>newAttr</var>'s
+ <a for=Attr>value</a>.[[!TRUSTED-TYPES]]
+
  <li><p><a for=list>Replace</a> <var>oldAttr</var> by <var>newAttr</var> in <var>oldAttr</var>'s
  <a for=Attr>element</a>'s <a for=Element>attribute list</a>.
 
@@ -6583,7 +6597,7 @@ string <var>namespace</var> (default null):</p>
 
 <div algorithm>
 <p>To <dfn export id=concept-element-attributes-set-value>set an attribute value</dfn> given an
-<a for=/>element</a> <var>element</var>, a string <var>localName</var>, a string <var>value</var>,
+<a for=/>element</a> <var>element</var>, a string <var>localName</var>, a string or <a>TrustedType</a> <var>value</var>,
 an optional null or string <var>prefix</var> (default null), and an optional null or string
 <var>namespace</var> (default null):
 
@@ -6594,7 +6608,7 @@ an optional null or string <var>prefix</var> (default null), and an optional nul
 
  <li>If <var>attribute</var> is null, create an <a>attribute</a> whose <a for=Attr>namespace</a> is
  <var>namespace</var>, <a for=Attr>namespace prefix</a> is <var>prefix</var>,
- <a for=Attr>local name</a> is <var>localName</var>, <a for=Attr>value</a> is <var>value</var>, and
+ <a for=Attr>local name</a> is <var>localName</var>, <a for=Attr>value</a> is stringified <var>value</var>, and
  <a for=Node>node document</a> is <var>element</var>'s <a for=Node>node document</a>, then
  <a lt="append an attribute">append</a> this <a>attribute</a> to <var>element</var>, and then
  return.
@@ -6862,7 +6876,7 @@ method steps are:
 
  <li><p>If <var>attribute</var> is null, create an <a>attribute</a> whose
  <a for=Attr>local name</a> is <var>qualifiedName</var>, <a for=Attr>value</a> is
- <var>value</var>, and <a for=Node>node document</a> is <a>this</a>'s <a for=Node>node document</a>,
+ stringified <var>value</var>, and <a for=Node>node document</a> is <a>this</a>'s <a for=Node>node document</a>,
  then <a lt="append an attribute">append</a> this <a>attribute</a> to <a>this</a>, and then return.
 
  <li><p><a lt="change an attribute">Change</a> <var>attribute</var> to <var>value</var>.