Companies like Yahoo need to commit to supporting standards, not browsers. Then, if the browser flavor of the week just does what the standard says, there's no problem. The users just pull out any compliant browser they want and know it will work.

Sadly, the market is dominated by one non-compliant browser, which means there's little financial incentive for websites to do anything other than make it work for the 800 lb. gorilla, standards or not. However, a good compromise would be to design to the standard, tweaking as necessary for IE to work. That way all other browsers would work fine if they followed the standards, and the website would deviate as little as necessary to give the IE-using masses the support they need. If other browsers ever put a dent in IE's market share, there would be some pressure on MS to fix IE's non-compliance.