<div id='content'>

  <form name='signup' id='signup' action='index.php' method='POST'>
    <input type='hidden' name='page' value='calendar' />
    <input type='hidden' name='own' value='<?php echo $experiment->owner; ?>' />
    <input type='hidden' name='exp' value='<?php echo $experiment->id; ?>' />

    <h2>Your details</h2>
    <p>Please input your details in the boxes below. If you leave your phone number, you will receive a text reminder on the day of the experiment. Your details will only be used for the purposes of this experiment.</p>

    <p><input type='text' name='name' size='30' id='name' placeholder='full name' value='<?php echo $_REQUEST['name']; ?>' /></p>
    <p><input type='text' name='email' size='30' id='email' placeholder='email' value='<?php echo $_REQUEST['email']; ?>' /></p>
    <p><input type='text' name='phone' size='30' id='phone' placeholder='mobile number (optional)' value='<?php echo $_REQUEST['phone']; ?>' /></p>

    <h2>Requirements</h2>
    <p>Please indicate that you meet the requirements for this experiment.</p>
    <?php $experiment->printRequirementCheckboxes(); ?>

    <p><input type='submit' id='button' name='signup_button' value='Next' /></p>
  </form>

</div>

<div id='content'>

  <form name='signup' id='signup' action='index.php' method='POST'>
    <input type='hidden' name='page' value='calendar' />
    <input type='hidden' name='own' value='<?php echo $experiment->owner; ?>' />
    <input type='hidden' name='exp' value='<?php echo $experiment->id; ?>' />

    <h2>Your details</h2>
    <p>Please input your details in the boxes below. If you leave your phone number, you will receive a text reminder on the day of the experiment. Your details will only be used for the purposes of this experiment.</p>

    <p><input type='text' name='name' size='30' id='name' placeholder='full name' value='<?php echo $_REQUEST['name']; ?>' /></p>
    <p><input type='text' name='email' size='30' id='email' placeholder='email' value='<?php echo $_REQUEST['email']; ?>' /></p>
    <p><input type='text' name='phone' size='30' id='phone' placeholder='mobile number (optional)' value='<?php echo $_REQUEST['phone']; ?>' /></p>

    <h2>Requirements</h2>
    <p>Please indicate that you meet the requirements for this experiment.</p>

    <div id='reqs'><?php $experiment->printRequirementCheckboxes(); ?></div>

    <p><input type='submit' id='button' name='signup_button' value='Next' /></p>
  </form>

</div>

Put requirement checkboxes in div
Put requirement checkboxes in div

The Javascript can then highlight them in red, if one or more hasn’t
been checked.
