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HTML5 is only one part of the equation, namely view. If your mid tier, backends etc are crappy, the end application will still be crappy, even if it looks pretty to the user. If you want to fix a poorly-programmed application (including but not limited to performance issues, scalability issues etc), you need to fix it in all layers, not just the view. So a blanket statement 'If HTML5 can make it easier for programmers to write code that works in Windows and on a tablet or phone ..' is incorrect. The right thing to think about is ...'Will It will help developers write device agnostic views'.
As Paul Thorott points out in his description of Microsoft Windows Live SkyDrive, rewriting applications to become HTML5-compliant can yield vast performance improvements as well as allow developers to add more modern features.
More likely, apps that can be sold through new app marketplaces (Apple, Android, Amazon, Microsoft) will be where we find new programs to replace aging vertical market apps. Since programmers respond favorably to the financial incentives, the promise of a much larger user base should appeal to their baser instincts. The biggest downside to HTML5 is that people don’t associate HTML with spending money.