css3 背景向两侧线性渐变

#demo4{
  line-height: 54px;
  letter-spacing: 4px;
  padding:26px 160px;
  color:#fff;
  background-image:linear-gradient(to right,rgba(0,0,0,0),rgba(0,0,0,0.1),rgba(0,0,0,0.2),rgba(0,0,0,0.3),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.4),rgba(0,0,0,0.3),rgba(0,0,0,0.2),rgba(0,0,0,0.1),rgba(0,0,0,0))
}
#demo4 span{
  text-transform: uppercase;
  text-shadow: 0 0 28px #000;
}

  一段简单的文字内容，测试用的

待优化点：linear-gradient 属性中加入了大量重复样式