USA reg's only require visiblity for landing minimums on a CAT 1 ILS, cieling doesn't matter.A FAR 121 airline pilot outside of the Final Approach Point, (glide slope intercept on a ILS}or Final Approach Fix , (non-precison app)cannot begin a approach if the reported visiblity is below the minimums of the chart. However, once a pilot is inside the Final Approach Point or Final Approach Fix, he may continue the appraoch even if the reported visiblity is below minimums. At DH he can then land if the runway enviroment is in sight or may have to go around if the visibilty is in fact below minumums and he can not see the runway enviroment. If ATC is reporting visibiliy at 0/0, then I don't know if you can start a approach. There are Cat III ILS's that permit very low visibilty landings and use a autoland feature.