<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
  <head>
    <title>JavaScript</title>
    <meta http-equiv="content-type" content="text/html; charset=iso-8859-1" />
    <script type="text/javascript">
    function pruefeFormular(f) {
      var element;

        {
        enr = new Array( "Fehler! ", " nicht eingegeben.", "Name", "Passwort", "Kommentar", "Land", "Altersgruppe", "Interessen", "Nutzungsbedingungen")
        }

      for (var i=0; i < f.elements.length; i++) {
        element = f.elements[i];
        switch (element.type) {
          case "text":
          case "password": 
          case "textarea":
          case "select-one":
            if (element.value == "") {
                alert(enr[0] + enr[i+2] + enr[1])
              return false;
            } 
            break;
          case "checkbox": 
            if (!element.checked) {
                alert(enr[0] + enr[8] + enr[1])
              return false;
            }
            break;
          case "select-multiple":
            if (element.selectedIndex < 0) {
                alert(enr[0] + enr[7] + enr[1])
              return false;
            }
            break;
          case "radio":
            var gruppenname = element.name;
            var gruppe = f.elements[gruppenname];
            var gruppe_ok = false;
            for (j=0; j<gruppe.length; j++) {
              if (gruppe[j].checked) {
                gruppe_ok = true;
              }
            }
            if (!gruppe_ok) {
                alert(enr[0] + enr[6] + enr[1])
              return false;
            }
            break;
        }


      }

    }

    </script>
  </head>
  <body>
  <form onsubmit="return pruefeFormular(this);">
    Name: <input type="text" name="Name" /><br />
    Passwort: <input type="password" name="Passwort" /><br />
    Altersgruppe:
      <input type="radio" name="Alter" value="minderjaehrig" />0-17
      <input type="radio" name="Alter" value="volljaehrig" />18-99<br /><p>
    Land: <select name="Land">
      <option value="">-- bitte w&auml;hlen--</option>
      <option value="D">Deutschland</option>
      <option value="A">&Ouml;sterreich</option>
      <option value="CH">Schweiz</option>
      <option value="sonst">sonstige L&auml;nder</option>
    </select><br /></p>
    Interessen: <br><select name="Interessen" multiple="multiple" size="3">
      <option value="HTML">(X)HTML</option>
      <option value="CSS">CSS</option>
      <option value="JS">JavaScript</option>
    </select><br />
    Kommentar: <br><textarea cols="30" rows="5" name="Kommentar"></textarea><br />
    <input type="checkbox" name="Bedingungen" value="ja" />
    Ich akzeptiere die Nutzungsbedingungen<br />
    <input type="submit" name="Submit" value="Registrieren" />
  </form>

  </body>
</html>
...zur Antwort