the alt tag will generate a roll over effect in all browsers. that is not the purpose if the alt tag code.
the purpose of the alt tag code is to identify the button on the page for screen reader devices for accessibility purposes.
eCart does not support a roll over cart total, if you want to show a running tally of the cart contents on the page, you can add quick cart display using the Display manager. this will show quantity, total, a view cart button and s checkout button.