The problem is caused by overlapping images and text. Standard HTML does not support overlapping items, so for each overlapping item an image is created. Thus there will be subtle differences in the display between these generated images and normal plain text.
Ensure that your text items do not overlap each other, and do not overlap any images.
It is important that no part of any text item - including its surrounding rectangle - overlaps anything else. When you select an item, the handles that appear around it indicate the size of this rectangle.
Alternately, you can build your site as Dynamic HTML, which does support overlapping items.