A paper in the most recent issue of British Journal of Ophthalmology looks at this. The authors searched for websites (they don't detail which sites they found), using a general search engine. They rated the information on its currency, author qualification, attribution and other things (according to various criteria including those from Health on the Net). Sites could be rated excellent, very good, good, fair, poor, and the majority of sites were fair or poor.
This brings out the need to evaluate websites, and also to use search tools that introduce an element of quality control - Health on the Net, OMNI, NMAP, and the like.
Read the paper (NHS Athens needed, if you are NHS staff; or University Athens if you are University and off campus).