They address this and tell you your browser is broken. But also that if you are using a non-broken browser and it's still hard to read for you then that's on purpose, they want it to be hard to read for you, because their content is for healthy people only.
they don't actually go into any detail how it's supposed to render, though. they refer to the standard but don't actually say what the browsers are supposedly doing wrong. It's not that the shadow being applied makes the text blurry somehow, it's that the shadow itself makes the text look like it's blurry, because it's very subtle. So, I don't buy it.
