No plugins, period. Microsoft IE lead Dean Hachamovitch:
Running Metro style IE plug-in free improves battery life as
well as security, reliability, and privacy for consumers.
Plug-ins were important early on in the web’s history. But the
web has come a long way since then with HTML5. Providing
compatibility with legacy plug-in technologies would detract
from, rather than improve, the consumer experience of browsing
in the Metro style UI.
There is no room in the future for Flash.