Created attachment 183655 [details]
mi mathbackground testcase.html
Steps to reproduce:
1. Create dynamically <mi> element
2. Assign mathbackground attribute
3. Append <mi> to <math>
background color does not render
background color should be rendered as per the mathbackground attribute
I was able to test it only on iOS 6.01.
So I guess it is the same issue as Mozilla's one: you must create the MathML elements with createElementNS + the MathML namespace, not simply createElement.
Created attachment 217791 [details]
testcase with explicit MathML namespace
Here is a version with createElementNS and it does work for me.