Still an issue in Safari 15.5. Its DevTools report:

Main thread

JavaScript: 0.0% (0)
Layout: 1.3% (9)
Paint: 98.7% (697)
Styles: 0.0% (0)

Energy Impact

Very High
Average CPU: 204.1%

After investigating, it looks like these are the culprits. When I turned off all of them in the Safari DevTools, the Average CPU dropped to 3.8%.

Code Elimination animation

  @keyframes pulse-red {
    0% {
      box-shadow: 0 0 0 0 rgba(255, 50, 82, 0.9);
    }

    100% {
      box-shadow: 0 0 0px 90px rgba(255, 50, 82, 0);
    }
  }

box-shadow is a notorious career performance criminal, especially when animated or transitioned. Animating transform and opacity on some pseudo-elements would probably be a good replacement.

Image placeholder popping in

.demo-page-lowres {
    filter: blur(1em) grayscale(0.6);
    opacity: 0.3;
  }

Maybe we should have two .demo-page-image elements, and toggle their opacity on and off instead, so Safari only needs to rasterize the filter once.

Demo progress bar

<div.demo-page-progress style={ transform: `scaleX(${Math.min(1, input.progress * (input.buffered ? 1 : 1/0.9))})` }/>

The worst part is this progress bar doesn’t even display right now — it’s missing either right: 0 or width: 100%. (I checked — not my fault in #63!)

It looks like doing the scaleX() dynamically in JS is flummoxing Safari. Slapping will-change: transform on .demo-page-progress made it much happier.

CPU load on https://markojs.com #101

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