Make pause-after and pause-before treat negative values as illegal
Make pause-after and pause-before treat negative values as illegal

svn path=/trunk/libcss/; revision=6239

prepare/netsurf_libcss,prepare/netsurf_libcss,servo/libcss,servo/libcss,servo/libcss,zhuyadong/libcss,zhuyadong/libcss,prepare/netsurf_libcss,zhuyadong/libcss