HTML form action and onsubmit issues
You should stop the submit procedure by returning false on the onsubmit callback.
<script> function checkRegistration(){ if(!form_valid){ alert('Given data is not correct'); return false; } return true; }</script><form onsubmit="return checkRegistration()"...Here you have a fully working example. The form will submit only when you write google into input, otherwise it will return an error:
<script> function checkRegistration(){ var form_valid = (document.getElementById('some_input').value == 'google'); if(!form_valid){ alert('Given data is incorrect'); return false; } return true; }</script><form onsubmit="return checkRegistration()" method="get" action="http://google.com"> Write google to go to google...<br/> <input type="text" id="some_input" value=""/> <input type="submit" value="google it"/></form>