<% ads.each_with_index do |ad, i| %>
  <div class="ad_excerpt_list">
    <% if ad_tab? %>
      <%= render "ads/actions", ad: ad %>
    <% end %>

    <%= render "ads/ad", ad: ad %>
  </div>

  <% if [5, 10, 15].include?(i) %>
    <div class="ad_excerpt_list">
      <%= render "partials/google_adsense", class_name: "desktop google_adsense_336_280", ad_slot: "7225596067" %>
      <%= render "partials/google_adsense", class_name: "mobile google_adsense_320_100", ad_slot: "1977811266" %>
    </div>
  <% end %>
<% end %>

<ul>
  <% ads.each_with_index do |ad, i| %>
    <li class="ad_excerpt_list">
      <% if ad_tab? %>
        <%= render "ads/actions", ad: ad %>
      <% end %>

      <%= render "ads/ad", ad: ad %>
    </li>

    <% if [5, 10, 15].include?(i) %>
      <li class="ad_excerpt_list">
        <%= render "partials/google_adsense", class_name: "desktop google_adsense_336_280", ad_slot: "7225596067" %>
        <%= render "partials/google_adsense", class_name: "mobile google_adsense_320_100", ad_slot: "1977811266" %>
      </li>
    <% end %>
  <% end %>
</ul>

Use a list tag for the list of ads
Use a list tag for the list of ads

That's what it is, so let's use the right semantics.
