mathie's bits

For some reason I have not been able to identify and test, this code would not work (show the same color or same color for subsequent rows, does not seem to have a pattern):

$("#tableid tbody tr:even").addClass("even");
$("#tableid tbody tr:odd").addClass("odd");

But this does work:

$("#tableid tbody tr:nth-child(even)").addClass("even");
$("#tableid tbody tr:nth-child(odd)").addClass("odd");

Any thought?