Since you have jQuery on the page, you can remove that onclick attribute and the majorpointsexpand function. Add the following script to the bottom of you page or, preferably, to an external .js file:
$(function(){
$('.majorpointslegend').click(function(){
$(this).next().toggle().text( $(this).is(':visible')?'Collapse':'Expand' );
});
});
This solutionshould work with your HTML as is but it isn't really a very robust answer. If you change your fieldset layout, it could break it. I'd suggest that you put a class attribute in that hidden div, like class="majorpointsdetail" and use this code instead:
$(function(){
$('.majorpoints').on('click', '.majorpointslegend', function(event){
$(event.currentTarget).find('.majorpointsdetail').toggle();
$(this).text( $(this).is(':visible')?'Collapse':'Expand' );
});
});
Obs: there's no closing </fieldset> tag in your question so I'm assuming the hidden div is inside the fieldset.