I am already following

https://wavesurfer.xyz/examples/?styling.js

It’s part of Shadow DOM.

Class regionHighlight if available then only we want to apply the class. There are multiple regions within waveform

HTML is <div id="waveform"></div>

So CSS will be like

#waveform {
  ::part(region) {

   &.is(.regionHighlight) {
    border: 2px solid red;
   }
  }
}

Above code is not working somehow. I am only able to give CSS via part only.