Hi Francesco,
because the landing pages of HR Renewal are based on SAPUI5, Internet Explorer 9 is the minimum browser version, which can be used to get an unrestricted feature support (more details see Browser Support in SAPUI5).
Besides, i would really recommend to use even a higher version of Internet Explorer e.g. 11. The performance and usability of the landing pages and UI5 apps in IE9 are not very satisfying. In my opinion, IE8 is not a sensible option.
Regards
Michael