postcardHTML = function (to_name, from_name) {
  return `<html>
  <head>
    <style>
      .postcard {
        width: 600px;
        height: 400px;
        background: #4285F4;
        text-align: center;
      }

      .postcard-text {
        font-family: Arial, sans-serif;
        font-size: 60px;
        font-weight: bold;
        text-transform: uppercase;
        color: #FFF;
        padding: 40px 0px;
      }

      .postcard-names {
        font-family: Monaco, monospace;
        font-size: 30px;
        text-align: left;
        color: #FFF;
        margin: 15px;
        padding-top: 5px;
        overflow: hidden;
        white-space: nowrap;
      }

      .postcard-footer {
        font-family: Monaco, monospace;
        font-size: 14px;
        color: #FFF;
        padding-top: 50px;
      }
    </style>
  </head>
  <body>
    <div class="postcard">
      <div class="postcard-names">
        To: ${to_name}
        <br>
        From: ${from_name}
      </div>
      <div class="postcard-text">
          Hello,<br>World!
      </div>
      <div class="postcard-footer">
        powered by Google
      </div>
      </div>
  </body>
</html>`;
}
Responds to an HTTP request from Cloud Tasks and sends an email using data
from the request body.

@param {object} req Cloud Function request context.
@param {object} req.body The request payload.
@param {string} req.body.to_email Email address of the recipient.
@param {string} req.body.to_name Name of the recipient.
@param {string} req.body.from_name Name of the sender.
@param {object} res Cloud Function response context.