您可以写像素而不是百分比:更新:更接近的纯色示例:
background: url(https://cdn.sstatic.net/stackoverflow/img/apple-touch-icon.png) repeat-x 0 0, -moz-linear-gradient(top, rgb(0,0,255) 0px, rgb(0,0,255) 154px, rgba(255,255,255,0.5) 155px, rgba(255,255,255,0.5) 300px);
background: url(https://cdn.sstatic.net/stackoverflow/img/apple-touch-icon.png) repeat-x 0 0, -webkit-gradient(linear, left top, left bottom, color-stop(0px,rgb(0,0,255)),color-stop(154px,rgb(0,0,255)),color-stop(155px,rgba(255,255,255,0.5)), color-stop(300px,rgba(255,255,255,0.5)));
background: url(https://cdn.sstatic.net/stackoverflow/img/apple-touch-icon.png) repeat-x 0 0, -webkit-linear-gradient(top, rgb(0,0,255) 0px,rgb(0,0,255) 155px,rgba(255,255,255,0.5) 155px,rgba(255,255,255,0.5) 300px);
background: url(https://cdn.sstatic.net/stackoverflow/img/apple-touch-icon.png) repeat-x 0 0, -o-linear-gradient(top, rgb(0,0,255) 0px,rgb(0,0,255) 154px,rgba(255,255,255,0.5) 155px,rgba(255,255,255,0.5) 300px);
background: url(https://cdn.sstatic.net/stackoverflow/img/apple-touch-icon.png) repeat-x 0 0, -ms-linear-gradient(top, rgb(0,0,255) 0px,rgb(0,0,255) 154px,rgba(255,255,255,0.5) 155px,rgba(255,255,255,0.5) 300px);
background: url(https://cdn.sstatic.net/stackoverflow/img/apple-touch-icon.png) repeat-x 0 0, linear-gradient(to bottom, rgb(0,0,255) 0px,rgb(0,0,255) 154px,rgba(255,255,255,0.5) 155px,rgba(255,255,255,0.5) 300px);
http://jsfiddle.net/pBVsD/12/