The <color-interpolation-method> specifies whether interpolation should use a rectangular color space or a polar color space with an optional hue interpolation method:

in <rectangular-color-space>
// or
in <polar-color-space>[ <hue-interpolation method>]

<rectangular-color-space>

One of the keywords srgb, srgb-linear, lab, oklab, xyz, xyz-d50, or xyz-d65.

<polar-color-space>

One of the keywords hsl, hwb, lch, or oklch.

<hue-interpolation-method> Optional

The algorithm for hue interpolation. It defaults to shorter hue.

<color-interpolation-method> = 
  in [ <rectangular-color-space> | <polar-color-space> <hue-interpolation-method>? ]  

<rectangular-color-space> = 
  srgb         |
  srgb-linear  |
  lab          |
  oklab        |
  xyz          |
  xyz-d50      |
  xyz-d65      

<polar-color-space> = 
  hsl    |
  hwb    |
  lch    |
  oklch  

<hue-interpolation-method> = 
  [ shorter | longer | increasing | decreasing ] hue  

The following example shows the effect of using different interpolation color spaces for linear-gradient().

HTML

<div>sRGB:</div>
<div class="gradient srgb"></div>
<div>Oklab:</div>
<div class="gradient oklab"></div>
<div>Oklch (with <code>longer hue</code>):</div>
<div class="gradient oklch-longer"></div>

CSS

.gradient {
  height: 50px;
  width: 100%;
}
.srgb {
  background-image: linear-gradient(in srgb to right, blue, red);
}
.oklab {
  background-image: linear-gradient(in oklab to right, blue, red);
}
.oklch-longer {
  background-image: linear-gradient(in oklch longer hue to right, blue, red);
}

Result

• <color>, <gradient>
• <hue-interpolation-method>