radial-gradient()

As with any gradient, a radial gradient has no intrinsic dimensions (en-US); i.e., it has no natural or preferred size, nor a preferred ratio. Its concrete size will match the size of the element it applies to.

To create a radial gradient that repeats so as to fill its container, use the repeating-radial-gradient (en-US) function instead.

Because <gradient>s belong to the <image> data type, they can only be used where <image>s can be used. For this reason, radial-gradient() won't work on background-color and other properties that use the <color> (en-US) data type.

向量漸變(Radial gradients)由其中心點、邊緣形狀輪廓及位置、色值結束點（color stops）定義而成。

To create a smooth gradient, the radial-gradient() function draws a series of concentric shapes radiating out from the center to the ending shape (and potentially beyond). The ending shape may be either a circle or an ellipse.

Color-stop points are positioned on a virtual gradient ray that extends horizontally from the center towards the right. Percentage-based color-stop positions are relative to the intersection between the ending shape and this gradient ray, which represents 100%. Each shape is a single color determined by the color on the gradient ray it intersects.

<position> (en-US)

The position of the gradient, interpreted in the same way as background-position (en-US) or transform-origin. If unspecified, it defaults to center.

<shape>

The gradient's shape. The value can be circle (meaning that the gradient's shape is a circle with constant radius) or ellipse (meaning that the shape is an axis-aligned ellipse). If unspecified, it defaults to ellipse.

<extent-keyword>

A keyword describing how big the ending shape must be. The possible values are:

Keyword	Description
closest-side	The gradient's ending shape meets the side of the box closest to its center (for circles) or meets both the vertical and horizontal sides closest to the center (for ellipses).
closest-corner	The gradient's ending shape is sized so that it exactly meets the closest corner of the box from its center.
farthest-side	Similar to closest-side, except the ending shape is sized to meet the side of the box farthest from its center (or vertical and horizontal sides).
farthest-corner	The default value, the gradient's ending shape is sized so that it exactly meets the farthest corner of the box from its center.

備註： Early implementations of this function included other keywords (cover and contain) as synonyms of the standard farthest-corner and closest-side, respectively. Use the standard keywords only, as some implementations have already dropped those older variants.

<linear-color-stop>

A color-stop's <color> (en-US) value, followed by an one or two optional stop positions (either a <percentage> (en-US) or a <length> (en-US) along the gradient's axis). A percentage of 0%, or a length of 0, represents the center of the gradient; the value 100% represents the intersection of the ending shape with the virtual gradient ray. Percentage values in between are linearly positioned on the gradient ray. Including two stop positions is equivalent to declaring two color stops with the same color at the two positions.

<color-hint>

Th color-hint is an interpolation hint defining how the gradient progresses between adjacent color stops. The length defines at which point between two color stops the gradient color should reach the midpoint of the color transition. If omitted, the midpoint of the color transition is the midpoint between two color stops.

radial-gradient(
  [ [ circle || <length> ]                         [ at <position> ]? , |
    [ ellipse || [ <length> | <percentage> ]{2} ]  [ at <position> ]? , |
    [ [ circle | ellipse ] || <extent-keyword> ] [at <position> ]? , |
    at <position> ,
  ]?
  <color-stop-list> [ , <color-stop-list> ]+
)
where <extent-keyword> = closest-corner | closest-side | farthest-corner | farthest-side
   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> | <length> ]

Gecko used to have a long-standing bug whereby radial gradients like radial-gradient(circle gold,red) would work, even though they shouldn't because of the missing comma between circle and gold. Also, calc (en-US) expressions were rejected — causing the value to be invalid — when used as the radius component of a radial-gradient() function (Firefox bug 1376019). Firefox's new parallel CSS engine (also known as Quantum CSS or Stylo, released in Firefox 57) fixed these bugs.

• Using CSS gradients (en-US)
• Other gradient functions: repeating-radial-gradient (en-US), linear-gradient (en-US), repeating-linear-gradient (en-US), conic-gradient (en-US), repeating-conic-gradient (en-US)
• <image> (en-US)
• image() (en-US)
• element() (en-US)
• image-set() (en-US)
• cross-fade (en-US)