Intro

div {
  width: 100px;
  height: 100px;
  background-image: linear-gradient(-45deg, peru, papayawhip);
  transition: 0.5s;
}

div:hover{
  background-image: linear-gradient(-45deg, peru, peru);
  transition: 0.5s;
}

What's wrong with this code? It should make a nice, clean, gradient animation, from a gradient to a solid fill. It should work, right?

Wrong

background-image is not animatable.

What's a developer to do? I lay in my bed and considered the possibilities. I could use Javascript DOM functions like backgroundImage and template literals to change the color. I could use a RGB to CMYK function, and call requestAnimationFrame. Or, I could look it up.

I found this.

What this StackOverflow thread suggests is basically increasing the background-size to "200% 200%" (I assume double the size of the target window) and then translate it using a bunch of percentages and background-position.

Cool! After spending a while reading up on documentation, I figured out this:

Assuming linear-gradient(145deg, red, blue), of course.

From the StackOverflow code and my newfound knowledge about background-position, I had my steps.

• Write gradient
• Increase gradient size
• Center gradient (50% 50%) so it looked about the same
• background-position is animatable, so translate either 100% 100% or 0% 0% to make it look more blue or more red, respectively

Yay! Problem solved. Here's a working demo:

But, as you can probably tell, it's not perfect - 100% 100% still has some red tinge to it, and on a blue background this div will definitely stand out. Also, combining a 200% background size and 50% background position changes the gradient slightly - as you can see in this side by side comparison:

What's a perfection obsessed developer like me to do? After excessive brainstorming, I figured out these things:

• background-size is animatable
• Changing the gradient to red, blue, blue makes a completely blue portion at one corner.

So my steps now looked like this:

• Start the div with a normal gradient *On hover, do these things:
  • change background-size to 200%
  • change background-position to 100% 100% (the blue blue part of the gradient)
  • use nicer colors than red and blue

While messing with the code, I found that I didn't need to change the background-size on hover; leaving it 200% to begin with actually helped differentiate the div from the same-colored background. I also added a quick box-shadow for a cool effect.

And here it is! A successfully animated linear-gradient, in pure CSS.

Helpful? Confusing? Thoughts? Please leave a comment!