The idea I want to convey is that all the resources corresponding to a web page will (most probably) be organized into a subtree, with the html page at the top of this subtree. Removing the top stream prevents the client of updating the relative priorities of two pages when the user changes the foreground tab. This may also be a problem for proxies to enable a fair sharing of the connexion between several clients (in this case it depends on how the proxy organize the streams of the different clients).

What about:

Related streams can be grouped inside a subtree of the dependency tree. Changing the weight of the root of a subtree affects the whole subtree. A peer can take advantage of this property to control the priority of a subtree relative to the other subtrees. Removing the root of the subtree makes it impossible to exploit this property.