chromium/chrome/browser/resources/lens/overlay/initial_gradient.html

<style>
  #scrimContainer {
    opacity: 80%;
  }

  #initialGradientScrim {
    background: linear-gradient(180deg,
        rgba(var(--color-scrim-rgb), 0.64) 0px,
        rgba(var(--color-scrim-rgb), 0.4) 104px,
        rgba(var(--color-scrim-rgb), 0) 100%);
    height: 250px;
    left: 0;
    opacity: 0;
    position: absolute;
    top: 0;
    transition: opacity 300ms ease-in-out;
    width: 100%;
  }

  :host([scrim-visible]) #initialGradientScrim {
    opacity: 100%;
  }

  :host([should-hide-scrim]) #scrimContainer {
    transition: visibility 0s;
    transition-delay: 300ms;
    visibility: hidden;
  }
</style>
<div id="scrimContainer">
  <div id="initialGradientScrim"></div>
</div>