zulip/static/templates/message_reaction.hbs

<div class="{{this.class}}" aria-label="{{this.label}}" data-reaction-id="{{this.local_id}}">
    {{#if this.emoji_alt_code}}
        <div class="emoji_alt_code">&nbsp:{{this.emoji_name}}:</div>
    {{else}}
        {{#if this.is_realm_emoji}}
        <img src="{{this.url}}" class="emoji" />
        {{else}}
        <div class="emoji emoji-{{this.emoji_code}}"></div>
        {{/if}}
    {{/if}}
    <div class="message_reaction_count">{{this.count}}</div>
</div>