well, it depends on whether the site breaks if a browser feature is missing or not.
if it doesn't break then you have a graceful fallback which i did already suggest as an acceptable way to support new features in case they are not supported.
but if the site breaks in some way, it may prevent me to complete the objective on the site. if i wanted to buy something, and the site doesn't work because of that missing browser feature, then i will have to go buy somewhere else. it doesn't matter how much leverage the site has if i can't use it.
if it doesn't break then you have a graceful fallback which i did already suggest as an acceptable way to support new features in case they are not supported.
but if the site breaks in some way, it may prevent me to complete the objective on the site. if i wanted to buy something, and the site doesn't work because of that missing browser feature, then i will have to go buy somewhere else. it doesn't matter how much leverage the site has if i can't use it.