[nbush]

If you want to take the effect to its hilarious conclusion, you can even do depth of field using SVG filters: http://nbush.github.io/depth/

[ygra]

Except that Gaussian blur does not approximate how we see things blurry outside the focal plane. Our pupils are not shaped like a bell curve with smooth falloff at the edges, instead they are round, thus requiring a different convolution kernel.