JSFiddle - React, Tailwind, and code Playground

HTML

<!doctype html>
<html lang="de" dir="ltr">
  <head>
    <meta charset="UTF-8" />
    <title>Simple jQuery Validation</title>
  </head>
  
  <body>
  
    <form id="simpleform" method="get" action="script.php">
      <input type="text" name="phone" placeholder="Telefon" />
      <input type="text" name="email" placeholder="E-Mail" />
      <input type="submit" value="Speichern"></input>
    </form>
  
  </body>
</html>

CSS

@charset "UTF-8";

* { margin: 0 ; padding: 0 ; border: 0 none ; }
body { padding: 10px ; }
input { padding: 10px ; font: 14px arial ; border: 1px solid #c0c0c0 ; }
input:focus { border: 1px solid #000 ; }
[type="submit"]:hover { cursor: pointer ; background-color: #f7f7f7 ; }

.invalid { border: 1px solid #84392B ; }

JavaScript

// the document is ready
$( document ).ready( function( ) {
 
  // handle the form's submit-event
  $( '#simpleform' ).submit( function( e ) {
    
    var phoneIsEmpty = ! $.trim( $( '[name="phone"]' ).val( ) ) ;
    var emailIsEmpty = ! $.trim( $( '[name="email"]' ).val( ) ) ;
    
    if( phoneIsEmpty && emailIsEmpty ) {
      alert( 'Bitte eine Telefonnummer oder eine E-Mail Adresse eintrage, damit wir Sie kontaktieren können.' ) ;
      e.preventDefault( ) ;
    }
  } ) ;

} ) ;