It may be valid CSS for a web browser but is it valid CSS in an ePub2 or ePub3 which use
subsets of CSS2 and CSS3 respectively. I ran your snippet through the W3C online validator. For Epub3/CSS 3, it commented about the multiple vendor extensions (the -webkit-font-whatver lines), for ePub2/CSS2.1, it not only complained about the vendor extensions but added 3 errors (2 about settings that don't exist in CSS2.1, one that complained about the use of rem which not valid in CSS2.1. You may want to start using ePub3 if you need to make use of those features. OTOH, anyone who uses Times New Roman in an ebook is a hopeless romantic.