72dpi vs 100dpi for web designers?

The difference is nothing really. The reason they say it's 72dpi is because someone at the W3C decided it was a good number most likely. But there are some differences technically since we see the same quality of image but there is extra data that is unnecessary in the 100dpi image. That's about the only difference. But when you properly optimize images it doesn't really matter what the dpi is