Summary
The box-shadow property describes one or more shadow effects as a comma-separated list. It enables you to cast a drop shadow from the frame of almost any element. If aborder-radius is specified on the element with a box shadow, the box shadow takes on the same rounded corners. The z-ordering of multiple box shadows is the same as multiple text shadows (the first specified shadow is on top).
Box-shadow generator is an interactive tool allowing you to generate a box-shadow.
Values
inset ：<是否为内部阴影>
If not specified (default), the shadow is assumed to be a drop shadow (as if the box were raised above the content).
The presence of the inset keyword changes the shadow to one inside the frame (as if the content was depressed inside the box). Inset shadows are drawn inside the border (even transparent ones), above the background, but below content.
<offset-x> <offset-y> ：<阴影左右偏移量>
These are two <length> values to set the shadow offset. <offset-x> specifies the horizontal distance. Negative values place the shadow to the left of the element.<offset-y> specifies the vertical distance. Negative values place the shadow above the element. See <length> for possible units.
If both values are 0, the shadow is placed behind the element (and may generate a blur effect if <blur-radius> and/or <spread-radius> is set).
<blur-radius> ：<边缘虚化程度>
This is a third <length> value. The larger this value, the bigger the blur, so the shadow becomes bigger and lighter. Negative values are not allowed. If not specified, it will be0 (the shadow's edge is sharp). The specification does not include an exact algorithm for how the blur radius should be calculated, however, it does elaborate as follows:
…for a long, straight shadow edge, this should create a color transition the length of the blur distance that is perpendicular to and centered on the shadow’s edge, and that ranges from the full shadow color at the radius endpoint inside the shadow to fully transparent at the endpoint outside it.
<spread-radius> ：<边缘伸缩长度>
This is a fourth <length> value. Positive values will cause the shadow to expand and grow bigger, negative values will cause the shadow to shrink. If not specified, it will be0 (the shadow will be the same size as the element).
<color> : <边框颜色>
See <color> values for possible keywords and notations.
If not specified, the color used depends on the browser - it is usually the value of thecolor property, but note that Safari currently paints a transparent shadow in this case.
Live examples
Browser compatibility
原文地址：https://developer.mozilla.org/en-US/docs/Web/CSS/box-shadow