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>
PS：是否可用于任何元素，包括文本？？
栗子：
div {
background: linear-gradient(to right, red, orange, yellow, green, blue, indigo, violet);
}
