This tooltip is based on the one here: http://viget.com/inspire/css-pointer-events-and-a-pure-css3-animating-tooltip
Please note that the tooltip content immediately follows the tooltip link in the code, so make sure that it reads semantically (a question/answer format works well).
Specific tooltips will probably need custom styling, depending on context/content.
GOOD: CSS-only BAD: You may need to style left margin and width of individual tooltips
GOV.UK template vx.x or above