zulip/static/templates/compose_banner/compose_banner.hbs

<div
  class="compose_banner {{banner_type}} {{classname}}"
  {{#if user_id}}data-user-id="{{user_id}}"{{/if}}
  {{#if stream_id}}data-stream-id="{{stream_id}}"{{/if}}
  {{#if topic_name}}data-topic-name="{{topic_name}}"{{/if}}>
    {{#if banner_text}}
    <p class="banner_content">{{banner_text}}</p>
    {{else}}
    <div class="banner_content">{{> @partial-block}}</div>
    {{/if}}
    {{#if button_text}}
    <button class="compose_banner_action_button" >{{button_text}}</button>
    {{/if}}
    <a role="button" class="zulip-icon zulip-icon-close compose_banner_close_button"></a>
</div>