Many web resources provide mechanisms for subscribing to changes in their content. However, this change information is only made available via a polling mechanism.
A number of people have proposed mechanisms for subscribing to updates to pages or feeds, however, a missing element of all these proposals is a subscription mechanism. Sadly, this means that every site supporting this feature will likely end up with a different implementation.
Get Pingd pour la mise à mort de la syndication... (à coups de XRDS, d'Atom et de web hooks, tout un programme)