Hi,
That looks strange. There is some HTML of the add to cart button system directly at the top of the HTML of the page, before the <head> tag, and that should not happen.
I suppose that it's caused by something in the template and if you try with the default template of Joomla, you don't have the issue.
I see that you're using the old add to cart system.
Try to deactivate the "add to cart legacy" setting in the hikaShop configuration in order to use the new add to cart system.
That should circumvent the problem.