Targetting CiviCRM input field labels with css
You can use an attribute selector in css, for example


  label[for="middle_name"] {   display:none; }


For more info see:
http://www.w3.org/TR/CSS21/selector.html#attribute-selectors
or
https://stackoverflow.com/questions/2599627/how-to-select-label-for-email-in-css

Does even work in IE, from version 8.