Hi Rhenando, thanks for the help first of all:
Here is the original link to my site, the one I'm animating is the "scroll to bottom" button on the right :
http://wthdesign.net/website/ideas/html/index.php
I think codepen isn't working properly in IE8, but I have uploaded the code as well for your debugging purposes:
http://codepen.io/vincentccw/pen/KaCiI
I have tried using the yoyo property like you mentioned, it does provide a better animation quality in IE8. The animation will repeat itself few times without any problems, but once it reaches a certain times it flickers for a second then back to normal again. (I think it is the best IE8 can handle )