zulip/web/templates/single_message.hbs

<div id="message-row-{{message_list_id}}-{{msg/id}}" data-message-id="{{msg/id}}"
  class="message_row{{#unless msg/is_stream}} private-message{{/unless}}{{#include_sender}} include-sender{{/include_sender}}{{#if mention_classname}} {{mention_classname}}{{/if}}{{#msg.unread}} unread{{/msg.unread}} {{#if msg.locally_echoed}}locally-echoed{{/if}} selectable_row"
  role="listitem">
    {{#if want_date_divider}}
    <div class="unread_marker date_unread_marker"><div class="unread-marker-fill"></div></div>
    <div class="date_row no-select">
        {{{date_divider_html}}}
    </div>
    {{/if}}
    <div class="unread_marker message_unread_marker"><div class="unread-marker-fill"></div></div>
    <div class="messagebox">
        <div class="messagebox-content {{#if status_message}}is-me-message{{/if}}">
            {{> message_body}}
        </div>
    </div>
</div>