[nine_k]

The technique is cool to know. SVG has many non-obvious abilities.

But the effect, due to the way it's produced, is more like a hot air distortion, only without the faint shimmering. It's completely raster in nature, AFAICT, and is likely implemented as a GPU shader (which is good from the performance POV).

An effect more like an unsteady human hand could likely be achieved by oscillating nodes in the direction perpendicular to the curvature, and adding some random jitter to the control points.

[Hasnep]

I had to re-read the second paragraph to make sure you weren't saying that heat haze is a GPU shader effect. Using the phrase "in nature" really didn't help me!