(iphone) safari textarea font?, and measuring it?

Context:

The idea is to add a graphical annotation, beside (or above) specific lines of text within a html text area. This is done, by overlapping a "auto generated" div item, above the textarea.

However while the alignment is perfect on most browsers, the custom font used, does not translate over to safari. And hence, it becomes misaligned, after falling back on it.

So the 开发者_JAVA技巧question is:

Whats the font use in the mobile safari text area (to calculate the spacing from the left) and how to derive its position, at any point of the text in a line? (even if it is not an even letter spacing font).