While for native applications this [various dpi] may be applicable, for web sites it's not quite so e.g. just one DPI fits all. In fact, there is not simple way to specify images for different DPI.
In addition it doesn't really matter what DPI you use when exporting images from Photoshop. For instance all images shipped with Telerik's RadControls for ASP.NET Ajax are exported with the "traditional" 72 DPI, while Windows 7 uses 96 DPI.
On your next question -- adapting the Ribbon to various resolutions: that happens mostly out of the box. The ribbon has the ability to reflow / resize it's own content (buttons, dropdowns etc) to fit the available space. If not possible, it will collapse the containing group.
It is of course possible to design a skin optimized for certain resolution / device to maximize the user experience on that device, but that's another story.
And lastly, but not least, complex and intricate command interfaces are not suited for mobile (small form factor) devices, as the screen real estate is really not suited for them.
If you have further questions about the ribbon bar, or any other control for that matter, do contact us back.
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