Uhm... stupid question....
What makes those multiple fieldsets? Why do you have forms inside of forms and put all sorts of non-form stuff inside that center form?
Looks like taking something simple -- a simple login form, and turning it into a needlessly complex mess; as evident by the 30k of markup for 628 bytes of plaintext and four visible form elements. Do yourself a favor, throw that mess out and start over... and keep track of your form closes so you don't end up with an invalid nesting order.
Because 25 validation errors is an accomplishment given HTML 5's loosening of the structural rules to the point it's a miracle ANYTHING works.
If you want to talk JUST that form, swing a massive axe at all the pointless extra garbage tags in there like SECTION, the garbage scripting that turns it into a needlessly complex mess, and keep the form bits together.... and lands sake, fieldsets are for a SET of fields, NOT SINGLE FIELDS!!!... and stop playing stupid bandwidth wasting content hiding games as if pageloads are some sort of evil. Maybe if you didn't waste 30k of markup on 3k's job, you wouldn't need all that scripted nonsense and wouldn't have accessibility issues galore.
<form action="/events/register" method="POST" id="member_form">
To successfully complete registration you must answer all questions in one go.<br />
Complete within 15 minutes or you may lose the information entered.
<p>Please create a memorable password to complete sign up.</p>
<p>Your email address is: firstname.lastname@example.org</p>
<label for="password">Enter password:</label>
<input id="password" name="Member[password]" type="password" class="required" />
<label for="passwordConfirm">Re-enter password:</label>
<input id="passwordConfirm" name="Member[passwordConfirm]" type="password" class="required" />
<input type="checkbox" name="CheckboxGroup1" value="checkbox" id="chk_accept_terms_conditions" />
<label for="chk_accept_terms_conditions" class="checkBoxLabel">
<a href="">Agree to Terms & Conditions</a>
<input class="submit" type="submit" value="Sign In" />
<input name="invitation_id" type="hidden" value="4f958ff6e2b07401000008be" />
<input name="event_id" type="hidden" value="4f9582d0a184b8010000000a" />
<input name="member_id" type="hidden" value="4f95851ad364c05cb9b8576b" />