Created by: coliff
The thead rule is safe to remove -- it was a workaround for browsers that didn't conform to this part of the CSS 2.1 spec. (https://www.w3.org/TR/CSS21/tables.html#table-display)
In practice, this meant IE 6 & IE 7.
(thanks to @mattbrundage for this PR upstream for the main.css project (https://github.com/h5bp/main.css/pull/101)