why are all the strings of Netflix's css not valid?

Question

While looking at the code of some sites including Netflix and W3Schools, I noticed something odd: when looking at some elements (by clicking Inspect Element in Firefox Developer Edition), almost all of the CSS rules/strings were crossed out. Why would that be? I attached two screenshots, 1 shows many crossed lines and 2 shows what element I was expecting.1 2

Answer 1

The majority of those rules crossed out are fallbacks for specific browsers.

-webkit is Chrome and Safari, -moz is Firefox, -o is Opera and -ms for Internet Explorer.