<html lang="en-US">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width">
    <title>:required and :optional with generated content</title>
    <link href="https://fonts.googleapis.com/css?family=Josefin+Sans&display=swap" rel="stylesheet">
  </head>

  <body>
    <form>
      <fieldset>
        <legend>Feedback form</legend>

        <p>Required fields are labelled with "required".</p>
        <div>
          <label for="fname">First name: </label>
          <input id="fname" name="fname" type="text" required>
          <span></span>
        </div>
        <div>
          <label for="lname">Last name: </label>
          <input id="lname" name="lname" type="text" required>
          <span></span>
        </div>
        <div>
          <label for="email">Email address (include if you want a response): </label>
          <input id="email" name="email" type="email">
          <span></span>
        </div>
        <div><button>Submit</button></div>
      </fieldset>
    </form>
  </body>
</html>
body {
  font-family: 'Josefin Sans', sans-serif;
  margin: 20px auto;
  max-width: 460px;
}

fieldset {
  padding: 10px 30px 0;
}

legend {
  color: white;
  background: black;
  padding: 5px 10px;
}

fieldset > div {
  margin-bottom: 20px;
  display: flex;
  flex-flow: row wrap;
}

button, label, input {
  display: block;
  font-family: inherit;
  font-size: 100%;
  padding: 0;
  margin: 0;
  box-sizing: border-box;
  width: 100%;
  padding: 5px;
  height: 30px;
}

input {
  box-shadow: inset 1px 1px 3px #ccc;
  border-radius: 5px;
}

input:hover, input:focus {
  background-color: #eee;
}

input + span {
  position: relative;
}

input:required + span::after {
  font-size: 0.7rem;
  position: absolute;
  content: "required";
  color: white;
  background-color: black;
  padding: 5px 10px;
  top: -26px;
  left: -70px;
}

button {
  width: 60%;
  margin: 0 auto;
}

External CSS

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External JavaScript

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