The browser does not typically understand or honor dots-per-inch (DPI) encoded in images and will always render images in the browser at a 96 dpi resolution.
To do this, you first need to tell Photoshop the dpi (dotsperinch) of your computer display. Mine, for example, is 113.21. You can figure out yours with this handy.
For example, you may want to use dotsperinch (DPI) scaling on a control displaying a chart or other graphic so that it always occupies a certain percentage of the screen.