if I have a valid html fragment in a variable like so;
var responseText = 'Lorem <span>Ipsum</span>';
How can I find the $("span").html() of that responseText, in other words get the Ipsum?
Usually, if responseText represented an HTML element, you could have used $(responseText) to parse it. Here, it easiest to create a temporary element, and setting its html:
var dummy = $('<div />').html(responseText);
var text = dummy.find('span').text();
Working example: http://jsbin.com/ebeju4