I get solid #808080 using Firefox 1 or 2 on a 24-bit display.
Using Firefox 1.5.0.10 on a 16-bit display, xv tells me the background is a solid #808080 but the grey portion of the image is #788078 and #807c80. I believe xv is telling me the literal values displayed on screen while gimp is translating them to an #ffffff-based system, so that my values are the same as yours once that's taken into account.
So what it seems is happening is that the CSS-styled background colour is being simply truncated to fit into the screen's bit-depth while the image is being properly mapped to the correct colours and then dithered. (I wonder if dithering is strictly necessary on a 16-bit display. Can you turn it off?)
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Using Firefox 1.5.0.10 on a 16-bit display, xv tells me the background is a solid #808080 but the grey portion of the image is #788078 and #807c80. I believe xv is telling me the literal values displayed on screen while gimp is translating them to an #ffffff-based system, so that my values are the same as yours once that's taken into account.
So what it seems is happening is that the CSS-styled background colour is being simply truncated to fit into the screen's bit-depth while the image is being properly mapped to the correct colours and then dithered. (I wonder if dithering is strictly necessary on a 16-bit display. Can you turn it off?)