When working with the mobile web, screen size and pixel density play a very important role and sometimes may overcomplicate things, because a pixel is not a pixel anymore (see the article by Peter-Paul Kock available at http://www.quirksmode.org/blog/archives/2010/04/a_pixel_is_not.html).
With the advent of high-pixel density devices, the pixel itself is a relative unit and fixed layouts have similar challenges to liquid layouts. When dealing with a native app (hybrid or not) on density-independent pixel displays, the virtual pixel-scaling algorithm adapts your app to the screen. For instance, a 100 px width image scales as follows according to the pixel density:
1x, 100px
1.5x, 150px
2x, 200px
The worst result you can get is that the images used in the UI are pixelated. In order to avoid this issue, you can simply embed in your app twice the resolution images, and then scale them accordingly to the device pixel ratio.