mock-xblock-editor.underscore

<div class="xblock xblock-studio_view" data-runtime-version="1" data-usage-id="i4x:;_;_edX;_mock"
     data-init="MockXBlock" data-runtime-class="StudioRuntime" tabindex="0">

    <div class="mock-xblock">
        <h3>Mock XBlock Editor</h3>
        <div class="metadata_edit"></div>
        <textarea data-metadata-name="custom_field">Custom Value</textarea>
    </div>

</div>