Why does the canvas content become fuzzy?

I used html5 canvas to draw a image, it works well in browser but not in iphone 5s. The content becomes fuzzy and not clear at all. I guess the canvas content was zoomed. Anyone met this before? Thanks.

By the way , I just used canvas api to draw circles, text and lines, not a actually image file.

This sounds like a HTML5 problem are you sure this question is not best directed on stack overflow.