Apply Options > "Assign to" - "All Desktop" only to 1 chrome tab? I keep my task list with rememberthemilk.com and I would like to have keep that chrome tab opened at all spaces all the time. Is there a way to achieve this? 

Right now if I go to chrome and Assign to All Desktop for it I am seeing other chrome windows being brought to the front when I switch spaces.
This is not possible natively. The desktop assignments are on a per-app basis, and not customizable by window.

A workaround could be to use Fluid, whereby you could create an actual application for the web app. This application could be assigned to all spaces.