This is on OS X, version 10.10.3 (Yosemite).

As shown in the animated GIF, enabling the following CSS style (which is present for <progress> elements) causes the CPU usage to spike:

-webkit-animation: animate-stripes 5s linear infinite;

From the Chrome profiling tools, you can see that all of the <progress> elements are being redrawn continuously while this CSS is being applied. Once it is removed, the CPU usage goes back down to zero and the redraw ends.

Outright removing this style only affects the display of indeterminate progress bars, as they appear to be designed to have a "barber poll" animation.

Something should be done so that package developers who introduce the use of a <progress> element do not inadvertently burn CPU.