Security e.g. users shouldn't trust a client based HTML5 with any serious data collection.- I'm not sure why you say that. I do online banking all the time. It is possible to code something secure and tight, but I'll admit it's not easy. I think security will always be an issue, even with native apps.
Local Data - Storage is limited & it can be manipulated
- True, storage needs to be unlimited. As for manipulating it, this is also true of native apps. Without jailbreaking, I can manipulate a save file for almost any iPhone game. The security issue (again) depends on the developer, and how they protect the data.
Syncing offline apps - Determining which is the latest version etc
- Yup, that's a challenge too. But also a challenge native apps face. When I make a note on an offline iPhone in the notes app, and then edit the same note on my Mac, it creates a duplicate when the iPhone is back online. It should probably be up to the developer how to handle this, whether the app is native or HTML5.
If the browser vendors won't implement something, it won't get implemented. This has also lead to
- Yes, this I think is one of the biggest challenges. People buy a computer, it comes with a browser default, and they never change or update it. It's hard enough to get the W3C to add new features, let alone browsers to implement them, and even harder still to get users to update their browsers.
Format Incompatibilities e.g. try audio/video tags across the major browsers
- Yup, similar issue.