`parent_text_top` is used when vertical-align is 'text-top'. `parent_text_top` should be the top of parent's content area and content area is defined in http://www.w3.org/TR/CSS2/visudet.html#inline-non-replaced. But since I couldn't find how to extract the em-box of the font, I assumed that the `parent_text_top` is font-size of parent and `parent_text_bottom` is 0 (Line 736).