Why is the root directory on a web server "www" is traditionally put in "/var"?

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http://unix.stackexchange.com – Tuxfiles says the following about the Linux directory structure: /var: This directory contains variable data that changes constantly when the system is running. It then goes on to say that things like logs, mail and the spooler are put in that folder. Traditionally, (Apache, nginx) the directory will be at /var/www/. It doesn't seem to me like the ideal place to put a directory with files or otherwise content that is supposed to be almost permanent. Why is it traditionally put into /var? More subjectively, is this where it should ideally go, according to the directory structure? (HowTos)