with Safari 8.0.7 this page: https://www.optimalworkshop.com/chalkmark causes the corresponding Safari Web Content process to slowly consume memory until the host becomes unusable (at about 50 GB on this MacBook)
Have you tried a nightly build of Safari? (http://nightly.webkit.org) With my bleeding-edge Safari version, I don't see the memory growing (How fast does it grow on your machine?). The WebContent process seems stable at less than 70MB for now.
Cannot be reproduced on latest nightly, and is sporadic even on the production machine originally reported on. I will revisit the problem, but retract this report.