Safari (Webkit?) rendering bug. Open The timelines panel of the dev tools, record and watch "Layout & Rendering" during these tests.

In Safari, the CPU gets pegged due to infinite rendering when all the following are true:

  1. a transition is defined that is not instantaneous
  2. there is an alpha channel in the color that is exclusively between 0 and 1
  3. there is at least one color channel which is exclusively between 0 and 255

No lasting CPU load when you hover me. (I do not satisfy requirement #3)

Watch Layout & Rendering when you hover on me. As long as you stay hovered, there is an infinite rendering loop.

I also create a loop, but in my case, it never stops after the first hover unless you refresh the page (remove me from the DOM). This is because I also transition out of the shadow.