This bug is not 'by design'. I suspect it is being closed automatically and not by a member of the team. Please speak with Ray Bango about this bug before attempting to close it as 'by design' again.
When the rem unit is used for font-size in either the 'font' or 'font-size' declaration the entire rule is ignored and the user agent default fort is used.
In basic terms, the use of rems to set type sizes causes IE10 to not even see the full rule.
See http://www.minerbits.com/REM-test-2-GF.html for a full test case