According to Yahoo's service agreement, there is no recourse for people who have seizures from exposure to their blinking ads except to stop using their service:
A SMALL PERCENTAGE OF USERS MAY EXPERIENCE EPILEPTIC SEIZURES WHEN EXPOSED TO CERTAIN LIGHT PATTERNS OR BACKGROUNDS ON A COMPUTER SCREEN OR WHILE USING THE SERVICE. CERTAIN CONDITIONS MAY INDUCE PREVIOUSLY UNDETECTED EPILEPTIC SYMPTOMS EVEN IN USERS WHO HAVE NO HISTORY OF PRIOR SEIZURES OR EPILEPSY. IF YOU, OR ANYONE IN YOUR FAMILY, HAVE AN EPILEPTIC CONDITION, CONSULT YOUR PHYSICIAN PRIOR TO USING THE SERVICE. IMMEDIATELY DISCONTINUE USE OF THE SERVICE AND CONSULT YOUR PHYSICIAN IF YOU EXPERIENCE ANY OF THE FOLLOWING SYMPTOMS WHILE USING THE SERVICE: DIZZINESS, ALTERED VISION, EYE OR MUSCLE TWITCHES, LOSS OF AWARENESS, DISORIENTATION, ANY INVOLUNTARY MOVEMENT, OR CONVULSIONS.
This is ridiculous. It would be a simple matter for them to include a preference for non-blinking, non-strobing ads and backgrounds. Shame on Yahoo.