yes, i know, about Explorer, the point is that competition does exists now.
Competition also slowly convinced Microsoft to at least try to keep up with the HTML living standard rather than stagnating. Users of Windows 7 are eligible for Internet Explorer 11, which supports new web platform features reasonably well according to caniuse.com. Right now the biggest headache is Internet Explorer pre-10, such as the IE 8 used by Windows XP diehards.
But it might be financially sound to just ignore the market of users of IE on Windows XP. Here's my reasoning: Operating system holes render browser security meaningless. If Windows XP is no longer supported, Microsoft is no longer patching known vulnerabilities that allow a miscreant to install a keylogger or other backdoor. And if a machine has such malware, any payment credentials or other private information sent to or from that machine isn't secret. A criminal could compromise Windows XP on a customer's PC, copy the credit card number that a user keys in, and then make fraudulent charges to that account. If you don't let users of known insecure browsers make a purchase, you won't have to worry as much about chargebacks.