<p>This seems to be related to partial  <code>app/views/layouts/_search.html.erb</code> being included twice in app/views/layouts/map.html.erb:</p>
<pre><code><% content_for :header do %>
  <%= render :partial => "layouts/search", :locals => { :autofocus => false } %>
<% end %>

<% content_for :content do %>
  <div id="sidebar">
    <%= render :partial => "layouts/search", :locals => { :autofocus => true } %>

</code></pre>
<p>Header level form was originally (re-)introduced in <a class="commit-link" data-hovercard-type="commit" data-hovercard-url="https://github.com/openstreetmap/openstreetmap-website/commit/dd12e1de637d5242c8ac7ed933fd8a3f70a70367/hovercard" href="https://github.com/openstreetmap/openstreetmap-website/commit/dd12e1de637d5242c8ac7ed933fd8a3f70a70367"><tt>dd12e1d</tt></a></p>

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