border-radius: 1px 2px 3px 4px / 5px 6px 7px 8px;
is the same as
border-top-left-radius: 1px / 5px;
border-top-right-radius: 2px / 6px;
border-bottom-right-radius: 3px / 7px;
border-bottom-left-radius: 4px / 8px;
The order is
border-top-left-radius x
border-top-right-radius x
border-bottom-right-radius x
border-bottom-left-radius x
/
border-top-left-radius y
border-top-right-radius y
border-bottom-right-radius y
border-bottom-left-radius y
;