| There is no simple answer to this question, for several reason.
First, you must consider CSS in itself. CSS specs. 2.1 does not concern itself with how a renderer (read browser) will load/display a styled element.
Second, each browser implements loading, unloading, caching ecc... in a different way.
In my experience, browsers threat css style-sheets this way:
whenever a css selector is triggered by a node the browser queue the image for loading... in certain cases reloading the same image several times.
To reach a conclusion in this question, either find authoritative replies from browser vendor or test the browser yourself.
Just to make myself clear, Firefox 1.5 does not necessarily behave like Firefox 2 nor like IE5 or IE6 or IE7 ... ecc...
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