We offer some support for the previous version of the spec. In those cases you can add a media query to detect IE10+ and override some of the properties. It works well as a starting point for supporting the legacy grid. In our tests the positioning is working as expected, but the sizing units might need some tweaks.

For me the biggest issue with IE grid is the lack of support for auto-positioning of elements. If you don't rely on that for your design you can support IE10+ pretty well.