It's mostly working well on all the browsers I've tried, including iOS 5 Safari and iOS 7 Safari, and Chrome, FF on Windows 7. The only issue is some weirdness when it renders the annotation off-screen when displayed to the left and right side of the content, and the screen size can't accommodate the content and annotation horizontally.
It's a difficult problem, but a better solution might be to move the annotations inside the content area, in the upper left or right corners when the body isn't wide enough to hold the annotation on the left or right side of the content. There could be one line of content visible, then the annotation with the arrow facing up on the left side for the 'left' annotation and right side for the 'right' one.