英文解释为：直线的，线形的长度的<数>一次的，线性的
而css中正式这种意思，例如线性渐变（linear-gradient）
CSS linear-gradient() 函数用于创建一个表示两种或多种颜色线性渐变的图片。其结果属于<gradient>数据类型，是一种特别的<image>数据类型。linear-gradient( [ <angle>| to <side-or-corner>,]? <color-stop-list>)
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Definition of the gradient line List of color stops
where <side-or-corner>= [ left | right ] || [ top | bottom ]
and <color-stop-list>= [ <linear-color-stop>[, <color-hint>? ]? ]#, <linear-color-stop>
and <linear-color-stop>= <color>[ <color-stop-length>]?
and <color-stop-length>= [ <percentage>| <length>]{1,2}
and <color-hint>= [ <percentage>
栗子：
div {
background: linear-gradient(to right, red, orange, yellow, green, blue, indigo, violet)
}
