Changing this from return undefined to return val fixes the issue.

Nice find! It seems jsdom doesn't handle the logic for any gradient function (linear-gradient, radial-gradient, conic-gradient, and its repeating variant).

Hi, does someone work on this issue?

Some notes.

• gradients are not <color>, they are <image>.
  https://drafts.csswg.org/css-images-4/#image-values
  So, it's not a bug that parseColor doesn't handle gradients.

• cssstyle supports background-image, but it is not following the spec enough.
  https://developer.mozilla.org/en-US/docs/Web/CSS/background-image#formal_syntax

  background-image =
    <bg-image>#
  
  <bg-image> =
    <image> |
    none
  
  <image> =
    <url> |
    <gradient>
  
  <url> =
    <url()> |
    <src()>
  
  <url()> =
    url( <string> <url-modifier>* )  |
    <url-token>
  
  <src()> =
    src( <string> <url-modifier>* )
  

  But IIRC https://github.com/jsdom/cssstyle/blob/main/lib/properties/backgroundColor.js, cssstyle only supports kinda

  background-image =
    <bg-image>	// no multiple images
  
  <bg-image> =
    <image>	// does not support `none`
  
  <image> =
    <url>	// no <gradient>
  
  <url> =
    <url()>	// src() is not supported. However, none of the browsers currently support src()
  

• Along with background-image, we should add below (although they should be treated as separate issues)

  • border-image-source and it's shorthand border-image
  • list-style-image and it's shorthand list-style