HTML as a cross-platform GUI?

Context : This is for a software develop internally and used only by company's employ开发者_如何学编程ees. Employees have windows, linux or Mac operating systems for the moment. Next, they may want to have a light version on their handheld devices.

Pro : cross platform

Con : may be not cross browser

Pro : GUI code should be smaller.

Con : but data transfer (HTML, images, css, javascript...) could cause bandwith issues ? (internal server has roughly 128kbps upstream)

Pro : anyone with a browser can use it, no install required.

Con : security issues ?

...

Any enlightened opinions ?

EDIT :

1/ Sorry about acceptance rate, I didn't know I had to accept answers.

2/ Some more conext about the application follows :

The application is simply a database front-end, somewhat similar to django's admin interface, nothing especially sophisticated. Just forms,listings, graphs and charts.

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I did some work producing a quite rich GUI application using ExtJS and the HTML5 canvas 2D API. It was fairly easy, though a few things e.g. data streaming/caching from a server had to be designed with the limitations of a Web client in mind.

Regarding data, it's not hard to transfer all the JS libs/css/images when the web app is 'initialising' (and give them very long cache expiry times) and then just use fairly terse JSON/xml/etc to transfer data once you are running.

As for security, HTTPS connections are fine for authentication. Of course you have to consider security as you add features, just like with any networked application.

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It's also good to think what kind of controls your program will require because special controls in the web world can be pain in the ass. Sorry, for the rude expression.

Your question doesn't say what kind of application it is.. 3D graphics, music, word processing, wiki etc. They're quite different from each other and will definitely be major thing when choosing platform or technology.