This operates on the el param, before the 'undefined' check of the el param.
This currently breaks the tablesort code on wmf-deployment for colspan and sortable-bottom cases.
At the top of the function 'el' could potentially be undefined or a string, in which the member function getAttribute is not available and would raise an error.
Fixed in r82572.
Leaving fixme status since I haven't been able to reproduce the bug TheDJ describes and thus can't say for sure if it's fixed.
See also [https://bugzilla.wikimedia.org/show_bug.cgi?id=27608 bug 27608]