var gradient = "linear-gradient( 109.6deg,  rgba(0,0,0,1) 11.2%, rgba(11,132,145,1) 91.1% )";
            window.onload = function () {
      const duration = 1500
      const style = document.createElement('style');
      document.head.appendChild(style);
      style.sheet.insertRule(`
            body::after {
            opacity: 0; /* 让渐变背景慢慢显现 */
            background-image: ${gradient};
            transition: opacity ${duration}ms linear;
            position: absolute ;
            top: 0;
            left: 0;
            width: 100%;
            height: 100%;
            background-color: yellow;
            content:"";
            }
        `, style.sheet.cssRules.length);

      setTimeout(() => {
       document.body.offsetHeight;
       const style = document.createElement('style');
        document.head.appendChild(style);
        style.sheet.insertRule(`
        body::after {
            opacity: 1; /* 让渐变背景慢慢显现 */
            content:"";
            z-index: -1;
        }
    `, style.sheet.cssRules.length);
    console.log(2)
      }, 500)

      setTimeout(() => {
        document.body.style.backgroundImage = gradient
        const style = document.createElement('style');
        document.head.appendChild(style);
        style.sheet.insertRule(`
            body::after {
                display: none;
            }
        `, style.sheet.cssRules.length);
        console.log("done")
      }, duration * 2)

    };