I developed an extension for Chrome and Opera, but I'm not using the Stackexchange API, instead my extension accesses the page data http://stackexchange.com (using Ajax) and displays in the extension.
- Show pushs in toolbar (new score + new messages in inbox)
- Show inbox messages
- Show achievements data
- Don't save data (it just shows real-time "stackexchange-button" data)
- Don't modify pages
- Don't require data user authentication
- Don't use API
Does this infringe any Stack Exchange policy?