A UA stylesheet is provided by each book reading platform, and provides its default styles. You can't prevent them from being there; they just are. All you can do is override anything that you consider to be important.
A word of caution, though.... Some readers misbehave badly when you try to set the page or body padding/margins to non-default values. So when you zero the margin, do so on the
img tag, not the
body tag.
