HTML5 hard truth No. 2: Local data storage is limited
The local databases buried in your browser are one of the neater features that make it simpler for Web apps to cache data on your computer. But for anyone hoping to offer desktoplike data functionality in the browser, these databases can save bandwidth and improve performance. However, they won't give users the same power over their data that they're used to enjoying with desktop apps.
HTML5 data storage capabilities are certainly an important addition, but you still can't move stored data to another machine, make copies, back it up, or open it with a different app. It's all buried deep where the browser hides it.
In a sense, these databases are the worst of both worlds. You get all of the responsibility for hosting the database but none of the control.
Some of the latest browsers allow you to see which databases have been created on your machine, but this information is limited. Safari even lets you delete the database. But you can't browse the information or move it to another machine. The files aren't designed to move easily, although you can do it if you know where to look.
Nor can you dig into the files to see what is stored there. Sure, a programmer can take them apart, but only after studying the format and doing some hacking. They're not like spreadsheets or text documents that are easy to open with any editor, making the data less resourceful than it might otherwise be in a desktop app.
HTML5 hard truth No. 3: Local data can be manipulated
The user may not have control over the data, but the central website is also hampered when dealing with client data. Did the user switch browsers? Did the user switch machines? Many Web developers just toss up their hands and use the local data storage for caching short-term content. They can't let the user create much because of the problems of synchronization.
If a Web app is connected to the Internet, it can continually save data to the cloud. When it's offline, changes aren't always stored in the cloud. When someone switches browsers or uses a different machine, copies begin to proliferate and the difficulties of synchronization rear their head. To make matters worse, clocks themselves may be unsynchronized, making them unreliable for finding the latest saved data.