Un formulario para Jekyll: sin plugin ni servidor y compatible con GitHub Pages.

<form action="https://mailcontact.app/api/forms/your-form-id" method="POST" data-mailcontact-form lang="es" data-mailcontact-language="es">
  <input type="hidden" name="_language" value="es" />
  <fieldset>
    <legend>Contáctanos</legend>
    <label>
      Nombre (opcional)
      <input name="name" type="text" autocomplete="name" maxlength="200" />
    </label>
    <label>
      Correo electrónico
      <input name="email" type="email" autocomplete="email" maxlength="320" required />
    </label>
    <label>
      Mensaje
      <textarea name="message" rows="5" maxlength="5000" required></textarea>
    </label>
    <noscript>
    <div aria-hidden="true" style="position:absolute;left:-10000px;width:1px;height:1px;overflow:hidden;">
      <label>Sitio web
        <input name="company_website" type="text" tabindex="-1" autocomplete="off" />
      </label>
    </div>
    </noscript>
    <button type="submit">Enviar mensaje</button>
  </fieldset>
  <p data-contact-status role="status" aria-live="polite" aria-atomic="true"></p>
</form>
<script defer src="https://mailcontact.app/forms/v1.js" crossorigin="anonymous" referrerpolicy="no-referrer"></script>

<form action="https://mailcontact.app/api/forms/your-form-id" method="POST">
  <label for="name">Name</label>
  <input type="text" id="name" name="name" />

  <label for="email">Email</label>
  <input type="email" id="email" name="email" required />

  <label for="message">Message</label>
  <textarea id="message" name="message" required></textarea>

  <button type="submit">Send</button>
  <input name="company_website" type="text" tabindex="-1" autocomplete="off"
    style="position:absolute;left:-10000px" aria-hidden="true" />
</form>