Safari and Firefox browser compatibility

This PR isn't *quite* finished - there's still a bit more to be done for Firefox compatibility. OK to start reviewing, but don't merge until I say it's ready...
So admittedly these proportions are a frustrating hack, because the better solution Sara and I had worked out when doing some live iteration at the offsite relied on SVG `baseline-shift`, which isn't supported by Firefox. I expect to lay a few options on the table re: browser compatibility in a meeting with Howard, Brandon, and @nicolashery next week, so for now I will add constants with comments. I also expect to iterate some more with Sara, but she's away this week.