macOS iBooks Author Widgets Unicode

Hello.

I seem to have a problem with certain (the longer ones) unicode when I programmed my HTML widget for iBooks.

Some unicodes work. I am trying to get an algebraic 'x' in my widget, which is the normal italic 'x'. However, now it is just showing blank square boxes.

It works fine on my browser when I tested the HTML file.

I suppose this is a compatibility issue? Is there any way to get my algebraic 'x' on iBooks widget?

Thanks for your help