Ghost/ghost/admin/app/components/modals/email-preview.hbs

<div class="modal-content">
    <header class="modal-header gh-pe-header gh-pe-header-border" data-test-modal="preview-email" style="display:flex">
        <h2 class="f6 fw6">{{this.subject}}</h2>
        <div>
            <div class="gh-contentfilter gh-btn-group gh-pe-btn-group" style="display:flex;flex-grow:1;justify-content:center">
                <button class="gh-btn {{if (eq this.tab "desktop") "gh-btn-group-selected"}}" type="button" {{on "click" (fn this.changeTab "desktop")}}><span>Desktop</span></button>
                <button class="gh-btn {{if (eq this.tab "mobile") "gh-btn-group-selected"}}" type="button" {{on "click" (fn this.changeTab "mobile")}}><span>Mobile</span></button>
            </div>
        </div>
        <div class="gh-pe-close">
            <button class="close" href title="Close" type="button" {{on "click" @close}}>
                {{svg-jar "close"}}
            </button>
        </div>
    </header>

    {{#if (eq this.tab "mobile")}}
        <div class="modal-body modal-preview-email-content gh-pe-mobile-container">
            <div class="gh-pe-mobile-bezel">
                <div class="gh-pe-mobile-screen">
                    <div class="gh-pe-emailclient-sender">
                        <p>
                            <span class="strong">{{this.config.blogTitle}}</span> &lt;{{this.settings.membersFromAddress}}&gt;
                        </p>
                        <p><span class="dark">To:</span> Jamie Larson &lt;jamie@example.com&gt;</p>
                    </div>
                    <iframe class="bn gh-pe-iframe" {{did-insert this.renderEmailPreview}} sandbox="allow-same-origin"></iframe>
                </div>
            </div>
        </div>
    {{/if}}

    {{#if (eq this.tab "desktop")}}
        <div class="modal-body modal-preview-email-content gh-pe-desktop-container">
            <div class="gh-pe-emailclient-mockup">
                <div class="gh-pe-emailclient-sender">
                    <p>
                        <span class="strong">{{this.config.blogTitle}}</span> &lt;{{this.settings.membersFromAddress}}&gt;
                    </p>
                    <p><span class="dark">To:</span> Jamie Larson &lt;jamie@example.com&gt;</p>
                </div>
                <iframe class="gh-pe-iframe" {{did-insert this.renderEmailPreview}} sandbox="allow-same-origin"></iframe>
            </div>
        </div>
    {{/if}}
</div>