IE9 team:  IE9 is looking to be a big step forward in standards support and compliance, and it now looks like the newest version of all major browsers will provide strong HTML5 support.  Unfortunately, IE6, 7 and 8, which do not support HTML5, will likely remain in widespread use for years to come, and the widespread adoption of HTML5 will be complicated or possibly even postponed until these users of those older browsers can view HTML5 content properly.

Once IE9 is completed, why not provide high-priority updates to older browsers that adds IE9's rendering engine next to their standard engines, for use on pages with the HTML5 doctype?  This would offer a painless transition for individuals afraid of or uninterested in major upgrades and for corporate environments with software reliant on these older rendering engines, and could push forward the viable adoption of HTML5 by years.