Illegal? Quite possibly, although I doubt any trouble would come of it 99.99% ofthe time (a cease-and-desist at worst).
But I'd be inclined to consider it "ethical", depending on the circumstances of course. Stealing a page from yesterday's New York Times would be a bad thing. Mirroring an interesting page on the rise and fall of the Obscure Empire of 1300BC, which hasn't been updated in 3 years and whose domain is about to expire - that's completely different. Chances are the author simply forgot about it - or even died. The fact that the site was allowed to expire shows the author didn't care about the information being out there, or it would have been taken down. And the information contained on said website may be very useful for a few people. At that point I'd consider it positive karma to keep the page up for posterity. Of course, it becomes an exercise in statistics to figure out how ethical it may be - Probability-Author-Would-Object vs Usefulness sort of thing.
Of course, if you don't feel it *has* to be on the web, perhaps it would be better to simply make a personal copy. You could send it to any friends you want to see it, or mirror it on an unlinked-to website. That should fall under 'fair use' any way you look at it.
As far as legal liability goes though, I'd predict you're 99% safe unless you ignore a request to take it down.
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