I'm just a rookie at this but I will tell you what worked for me. Maybe it will work for you also.
I created an anchor just above my captcha area using your example: <a id="valid"></a>
Then double click on the WA Server Validations in the Server Behaviors panel to reopen the Server Validations: Insert window, at the bottom for Failed redirect: I entered: mypagename.php#valid this is the name of the page with the validations.
You also have to make the modifications suggested by Jason above. For me this code was at line 83