Okay but how do you compute an image? How would your browser -- or any other client software -- know what's the hero image of a blog that you never visited before, for example?

I feel like I am missing something important in your comment.

The article describes computational method of rendering frosted glass effect. You can achieve the same thing by rendering the effect once (then upload to a sever) and have client download the rendered image. Or you can compute the frosted glass effect. What's better? That's the argument.