Example
Markup: components/email-signup/email-signup-header.twig
<div style="display:block;" class="cr-header-email-signup-form contextual-region block-cr-email-signup block--cr-email-signup block--cr-email-signup--head block--cr-email-signup--step-1 crEMailSignup-processed" data-drupal-selector="cr-header-email-signup-form" id="block-emailsignupblockhead">
  <div class="block--cr-email-signup__inner-wrapper">
    <div class="cr-email-signup--messages">
      <div class="esu-title-message">
        <p>Stay in the know!</p>
      </div>
      <div class="esu-initial-message">
        <p>Get regular email updates and info on what we're up to!</p>
      </div>
      <div class="esu-first-success-message">
        <p>Thanks! Your first email will be with you shortly</p>
      </div>
    </div>
    <div class="esu-errors messages messages--error"></div>
    <form action="/web/" method="post" id="cr-header-email-signup-form" accept-charset="UTF-8" data-drupal-form-fields="edit-email,edit-step1">
      <input class="esu-device" data-drupal-selector="edit-device" type="hidden" name="device" value="OS_X_10.11.6_-_Chrome_56.0.2924.87">
      <input class="esu-source" data-drupal-selector="edit-source" type="hidden" name="source" value="Header">
      <div class="js-form-item form-item js-form-type-textfield form-type-textfield js-form-item-email form-item-email">
        <label for="edit-email">Your email address</label>
        <input class="–metrika-nokeys form-text" data-drupal-selector="edit-email" type="text" id="edit-email" name="email" value="" size="60" maxlength="500" placeholder="example@youremail.com">
      </div>
      <input class="step1 button js-form-submit form-submit" data-drupal-selector="edit-step1" type="submit" id="edit-step1" name="step1" value="Subscribe">
      <input data-drupal-selector="form-bja0x4d6a3lmjjlyotlzzlq-pazuc5phi7qyduqqcya" type="hidden" name="form_build_id" value="form-BJA0X4d6a3LmJJlyotLzzlQ_pAZuc5PhI7QyduQqCYA">
      <input data-drupal-selector="edit-cr-header-email-signup-form-form-token" type="hidden" name="form_token" value="BREegNF1jBbSdnIuntqc0VFn7apoecLt3Dc0ubP8pFs">
      <input data-drupal-selector="edit-cr-header-email-signup-form" type="hidden" name="form_id" value="cr_header_email_signup_form">
      <div class="esu-second-success-message">
        <p>Great! Now we know what's right for you.</p>
      </div>
    </form>
    <div class="esu-privacy-message"><p>Our <a class="link link--white inline" href="/privacy-policy">Privacy Policy</a> describes how we handle and protect your information.<br><br>If you are under 18, please make sure you have your parents’ permission before providing us with any personal details.</p></div>
    <a role="button" class="close-button" href="#">
      <svg class="icon">
        <use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#close-cross"></use>
      </svg>
    </a>
  </div>
</div>

Source: _shop19-styleguide.scss, line 374