what 'device' would that be -- it's hard to relate without a frame of reference.
IDEALLY when marking up the page you use semantic markup with obvious containers, then you can use CSS3 media queries to add/remove/refactor the layout for each potential target... with maybe some .js as a fallback, though I really stopped doing that.
That's really what you want to look into is CSS3 media queries -- some people advocate mobile first then building it up, but that falls apart on legacy desktops that don't know media queries giving IE8/lower users a really crappy narrow stripe. I prefer to make a semi-fluid desktop layout first, then use media queries to strip away the stuff that doesn't fit. Older handhelds that don't know media queries typically are used to dealing with people making desktop only layouts anyways as that's what they've always had to deal with.
Though a LOT of the crap people put on websites -- like the massive banners for nothing, idiotic image rotators, equal width columns -- all shoe-horn you into a fixed width layout in the first place; which on the whole is pissing on accessibility and destroying your chances of even targeting multiple devices in the first place. Fixed width == /FAIL/ at web design, anyone telling you otherwise is probably thinking "But I can do it in photoshop".