* Sites want to offer a way for users to opt into a standalone mode.
   
 * Sites want this mechanism to be inline so that they can position it.
  
 * This mechanism shouldn't be visible in user agents where the feature 
   isn't available.
 
 * This mechanism shouldn't be visible when the user has already activated
   the feature.

 * It would be better if, for the previous two cases, instead of just
   hiding the feature, it could be shown but disabled when not relevant.

 * This mechanism shouldn't depend on scripts.

 * It would be better if this mechanism could use user-agent specific
   iconology instead of site-specific iconology, so that users could learn
   to look for particular icons, as they have with RSS.

 * It would be better if this mechanism could integrate with the menu/
   command feature in HTML5.