With new CSS features it is common to see browser prefixes

- these are ways for browsers to support features not fully standardised.
- work round different browser implementations prior to being fully finalised.
- show that features are not a 100% complete in the CSS spec.

An example series of browser prefixes

-webkit-animation-fill-mode: forwards; /* webkit browser e.g. Safari & Chrome */
-moz-animation-fill-mode: forwards; /* Mozilla browser */
-ms-animation-fill-mode: forwards; /* Microsoft */
-o-animation-fill-mode: forwards; /* opera */
animation-fill-mode: forwards; /* un-prefixed example */

The unprefixed version is always last to ensure the code is future proof (once the css spec is finalised) as it will override the prefixes listed above it.