One anti-spambot measure I've seen is to run the javascript to change a link on mouseover. So while it's possible that spambots are using automated browsers (quite possible, I'd say) it's unlikely they are simulating the movement of the mouse around the screen.
Not sure what happens if you tab over such a link. Probably broken in that case.
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That's a very good idea and caveat you mentioned about the tab is, just like I argued, tough. I'll keep that one in mind.
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One anti-spambot measure I've seen is to run the javascript to change a link on mouseover. So while it's possible that spambots are using automated browsers (quite possible, I'd say) it's unlikely they are simulating the movement of the mouse around the screen. Not sure what happens if you tab over such a link. Probably broken in that case.