You only need to write the unprefixed version, and the framework automatically adds any prefixes that are necessary when generating the CSS output. If you choose to add them, they are used as-is. This enables you to specify alternative values for certain prefixes.
For example, this is an unprefixed version of border-radius.
.class { border-radius: 2px; }
The previous declaration results in the following declarations.
.class { -webkit-border-radius: 2px; -moz-border-radius: 2px; border-radius: 2px; }