That's a very good point although I think using last 2 major versions, not dead vs a % might be a more robust alternative. That will remove older browsers that aren't supported any longer as well as include the latest versions regardless of market share.
last 2 major versions, not dead
Too much targetting of old browsers and you end up with a lot more code than is feasible. Sometimes its just better to let users know that old, insecure browser is not supported.
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