I don't doubt your explanation, but it's tragic and backwards. It would make sense that a WordPress recipe blog wouldn't be developed with smaller browsers in mind. But 5% of Instagram's traffic is a gigantic amount of traffic, worth catering to. Wouldn't it be the big websites that have both the budget and the motivation to make their sites work across browsers?

Call me a luddite, but I think if I were the website director I'd be saying "If you have to do something special to make it work in Browser X vs Browser Y, you're getting too fancy, you don't need that crap."