Google announced a plugin that allows people to write online applications that work offline. In other words: offline applications that work online. (Google Gears API Developer's Guide (Beta)) The APIs provide you with ways to tell which resources need to be cached if the application is to be used offline (and also allows developers to query whether a resource has been cached or not), with a local storage database (based on SQLite) and with a way to execute JavaScript asynchronously. Erik from Google posted about making it an open standard and Maciej from Apple started discussion about Gears on the WHATWG mailing list. The next step is looking into how it integrates with the features HTML5 already has and making it work well together with the rest of the web application platform. After all, supporting offline applications makes perfect sense. I assume that during that process the features will be added to the HTML5 proposal and browsers can start natively implementing them in so far they are not already doing that. (For instance, the current HTML5 proposal already has client-side SQL database support.)
Have a look at http://code.google.com/apis/gears/ samples HTML code, isn't it trying to be some kind of HTML5? :p Doesn't validate, though.