diff --git a/source b/source
index ab7fe6a890d..7180d7868aa 100644
--- a/source
+++ b/source
@@ -103201,8 +103201,8 @@ dictionary <dfn>StorageEventInit</dfn> : <span>EventInit</span> {
 
    <li><p>Let <var>local name</var> be the tag name of the token.</p></li>
 
-   <li><p>Let <var>is</var> be the value of the "<code data-x="attr-is">is</code>" attribute in the
-   given token, if such an attribute exists, or null otherwise.</p></li>
+   <li><p>Let <var>is</var> be the value of the first "<code data-x="attr-is">is</code>" attribute
+   in the given token, if such an attribute exists, or null otherwise.</p></li>
 
    <li><p>Let <var>definition</var> be the result of <span data-x="look up a custom element
    definition">looking up a custom element definition</span> given <var>document</var>, <var>given
@@ -103251,8 +103251,9 @@ dictionary <dfn>StorageEventInit</dfn> : <span>EventInit</span> {
    </li>
 
    <li>
-    <p><span data-x="concept-element-attributes-append">Append</span> each attribute in the given
-    token to <var>element</var>.</p>
+    <p>For each attribute in the given token, if <var>element</var> does not already have an
+    attribute with the same name, <span data-x="concept-element-attributes-append">append</span> the
+    attribute to <var>element</var>.</p>
 
     <p class="note">This can <span>enqueue a custom element callback reaction</span> for the
     <code data-x="">attributeChangedCallback</code>, which might run immediately (in the next