CSS background:linear-gradient(red,yellow); not working

[rja907]

I just tried to use this library to generate PDFs using HTML templates.
Here is some code:
CSS

.thank-you-background {
  height: 20px;
  background: linear-gradient(red, yellow);
}

HTML

<div class="thank-you-background">
   <p><strong>THANK YOU FOR YOUR BUSINESS!</strong></p>
</div>

I have also tried using background-image: linear-gradient(red, yellow); but that gave an explicit error saying com.openhtmltopdf.css-parse WARNING:: (./#inline_style_1) Value for background-image must be an identifier or a URI at line 76. Skipping declaration..

The HTML renders it correctly but the library does not show the gradient background.

[VictorAtPL]

I might be wrong, but linear-gradient looks to be introduced in CSS3 specification.

Open HTML to PDF is a pure-Java library for rendering arbitrary well-formed XML/XHTML (and even HTML5) using CSS 2.1 for layout and formatting, outputting to PDF or images.

This is from overview section in this repo's readme file.

There are supported some CSS3 features, like rotate or columns, but linear-gradient looks not to be implemented. I'd suggest just using image background.