Far from being "free" unfortunately. Especially if it is made on CPU side. But modern browsers are trying to use GPU for that. GPU has a primitive operation: render triangle with three colors at corners. So for non-trivial gradients you will need (on CPU side) to do so called triangulation to produce meshes containing such triangles: http://www.svgopen.org/2011/papers/18-Advanced_Gradients_for.... And then you need to send that mesh to GPU for batch rendering. Quite complex machinery.

Flash and Java were trying to do all that on CPU side. But with high DPI monitors (like 300 PPI Retina - 9 times more pixels on screens) CPU rasterization is not the option anymore.

https://www.chromium.org/developers/design-documents/gpu-acc...

(I actually spoke with them personally about this while I was still at Google, but there's the design doc. It's been 2 years, so perhaps the internals have changed, but this was also the situation in WebKit at the time as well.)

And you do see weird pixelation effects - try rendering some text to a div and then animating 'transform: scale()'. You lose font antialiasing during the animation, which is why many sites that do this will fade out the text first, animate only the bounding box, and then fade the text back in.

Here is what happens in my Sciter: http://sciter.com/screenshots/css-transform.png

As you see the rendering is made in vector form, not as a bitmap rotation.

There was a project to go all-GPU rendering within Chrome when I left, but it was a big project, and I don't know if they ever finished it. Even if they did, that still doesn't speak for WebKit/Edge/Firefox/Servo. (Edge is usually pretty modern when it comes to GPU acceleration, and I've heard Servo is heading this way, but Apple has historically cared very little about animation performance on WebKit, instead hoping to drive people to write native iOS apps.)

This isn't really an accurate description of the current state of affairs.

Edge, Firefox, Chrome, and Safari all use essentially the same architecture for rendering. In the case of all browsers but Chrome and Firefox on non-Windows platforms, they are already doing all their rendering using vector graphics APIs that are GPU-assisted (Direct2D for Edge and Firefox; CG::OGL for Safari); at least on Windows, Chrome is the last one to catch up here. All browsers use a "paint"/"compositor" distinction, which leads to the "transform/opacity is fast, other CSS properties are slow" effect. In the case of Safari, the compositor is run by the OS window server (Core Animation); Chrome is heading this way too on Mac. For other browsers and OS's, the compositing is done by the browser itself.

This design, which essentially all shipping browsers share, is motivated not by Web compatibility concerns but rather because it was easy to retrofit onto older browser engines. Servo does things differently: it removes the distinction between painting and compositing, replacing it with WebRender, an integrated solution that handles both. This means that it can run animations off the main thread and GPU-accelerate all of them, not just transform and opacity. The legacy vector graphics APIs are cut out of the picture entirely (except for SVG/canvas), which results in a much more optimized graphics pipeline. We do not break the Web while doing so; there is nothing in the spec that requires the use of legacy vector graphics APIs when painting Web content.

The only case when transformation are made on bitmap buffers is then 3D transformations are involved. Everyone knows how to do per-primitive 2D transformations these days.

https://jsfiddle.net/nqaoq52n/3/

If I run it through Chrome's Timeline inspector, the majority of the time in each animation frame is spent on the GPU (0.32 ms), while Paint only takes 0.01ms and Composite Layers is 0.03ms (there're also fairly lengthy 0.13ms Recalc Styles and 0.22 Update Layer Tree phases, where I suspect it's walking the whole JSFiddle DOM to find out nothing has changed).

If I run it through Firefox's profiler, there is a lengthy Paint (~8ms), followed by only Recalc Styles per animation frame (I'm guessing that it's doing everything on the GPU and the profiler doesn't show me that work), except when the element grows enough to force a scroll bar to appear, which trigger a re-layout.

No browser does this.

http://www.keithclark.co.uk/labs/css-fps/nojs/

Edit: The site seems to be overloaded. Here's an archive:

https://web.archive.org/web/20160702222211/http://keithclark...

For me, it doesn't run well on FF, but neatly in Chrome. Huge difference.

What a time to be alive.

Not strictly true; it's also possible to animate SVGs using ECMAScript or SMIL: https://www.w3.org/TR/SVG/animate.html

The difference is SVG + CSS is a better abstraction for creating illustrations than crazy CSS gradient and box shadow hacks.

That would depend on your browser. Because AFAIK there is nothing in the CSS spec that guarantees any minimum performance.

So don't count on it being fast!