[nitrogen]

On devices where 1px != 1 device pixel, images should be scaled by the same proportion as the px unit. So, if you set an img to 150px wide, and a div below it to 150px wide, they will be the same width, even if 150px == 300 pixels. I'd be happy to learn of any browsers where that is not the case.

[JoshTriplett]

If I wanted an image scaled, I'd give it a size in em or similar, not in px.

[nitrogen]

HTML is a text markup language, not a device interface language. CSS is a page layout language. HTML and CSS are designed for optimum user control as well as designer flexibility. You assume you're designing for one type of device. Designers before you have assumed 96dpi for web pages. The W3C realize these assumptions, realize that the user may want to use a different type of device, and thus define px in terms of DPI.

As others have mentioned, if a user prints a web page on a 1200dpi printer, your 960px fixed layout is 4/5ths of an inch wide. Your users would not be pleased. Under the device-centric interpretation of px, you'd have to provide a medium-specific style sheet for every possible type of output device. Designers would not be pleased.

Maybe you're trying to use HTML+CSS as a rendering target for a general purpose widget library like Gtk+. In that case, I can understand your frustration.