HTML

  
   <form action="">
     <p>Username: </p>
     <input type="text" pattern="[A-Za-z0-9]+" required>
     <input type="submit" value="Validar">
   </form>
  

CSS

  
    input {
      padding: 8px;
      margin: 1rem 0;
      border: none;
      box-shadow: black 0 0 4px;
    }