[alexquez]

Yea, CSS animations are insanely CPU heavy. CodePen has to kill them after 5 seconds to keep the site responsive.

[html5web]

Main page of codepen.io uses JS code to kill animations after 3-5 secs. When you click on single codepens it opens without JS animation killing restrictions.

[malandrew]

AFAIK a CSS3 3D transform of 0,0,0 with preserve 3d should push the work to the GPU.