I constantly find myself creating sites using newer technologies (such as HTML 5 and CSS 3, then adding either backwards compatibility layers or porting entirely to XHTML 1.0 and CSS 2.1. This process often involves replacing CSS with sprite images and adding in flash components for advanced asynchronous support.
My question is when can I can start to use the modern standards exclusively. Do I need to wait until Internet Explorer 9 is released or is it acceptable to request users switch to a "better" browser? Furthermore, if dropping compatibility for non HTML 5 browsers is an option, is it reasonable to ask mainstream users to install the Google Frame Frame?
Thanks.