Expected:
Given a <progress /> component, when its value property transitions from being a float to being null, I expect the value attribute of the <progress> tag to become unspecified, putting the control in the "indeterminate" state (the state with the animating barbershop pole pattern).
Actual:
When transitioning from being a float to being null, the value attribute of the <progress> tag becomes "0".
Test case:
http://jsfiddle.net/kb3gN/1981/