The favicon for ArXiv.org is a yellow smiley in front of 2 crossed bones on a green background. When I'm reading papers this icon appears everywhere on top of my browser tabs, but I have no idea of its meaning.
Thank you, I somehow missed that link. For reference: the crossbones refer to the "xxx" in the name of the original server (xxx.lanl. gov). The smiley and the green background don't seem to have any specific meaning though.