I was occasionally seeing weird baseline calculations in firefox that would then position the text wrongly

After a lot of debugging it seemed to come down to a strange situation in Firefox where the img.offsetTop property wouldn't be accurate immediately after it was appended to the dom, but would be updated if given a chance. Might have been some sort of threading issue? If you stepped through everything it worked fine, but running over the code and breaking on the next line produced this weirdness:

![Screen Shot 2013-04-26 at 3 35 18 PM](https://f.cloud.github.com/assets/79998/433114/eb2cd5e0-aebc-11e2-9771-9dac7eb1b36c.png)

I tried replacing the inserted img tag with a span to see if that would solve the problem and it seemed to. I also use font-size: 0px instead of img.height = 1, which should result in a 1px more accurate baseline reading.