If IE9 support is part of your project's requirements, you would just use Bootstrap 3 or one of the many alternatives that provide that as a feature. For a "new

If IE9 support is part of your project's requirements, you would just use Bootstrap 3 or one of the many alternatives that provide that as a feature. For a "new" framework (as these major revisions essentially are) it shouldn't matter as there are no users/victims (aside from testers).