opacity()

filter: opacity(1);

filter: opacity(80%);

filter: opacity(50%);

filter: opacity(0.2);

filter: opacity(0);

<section id="default-example">
  <img
    class="transition-all"
    id="example-element"
    src="/shared-assets/images/examples/firefox-logo.svg"
    width="200" />
</section>

opacity(0%)   /* 完全透明*/
opacity(50%)  /* 50% 透明 */
opacity(1)    /* 无效果 */