lucy24 - 12:14 am on Apr 21, 2011 (gmt 0)
What's the significance of "the latest major browsers"? Is there a direct correlation between the sites' content and their potential visitors' willingness and/or ability to use the most recent browsers? Does HTML 5 include features that are so attractive, its benefits would outweigh the risk of losing some proportion of users?
Treat those as serious, non-rhetorical questions for your clients to consider.