The <alpha-value> CSS data type represents a value that can be either a <number> or a <percentage>, specifying the alpha channel or transparency of a color.


The value of an <alpha-value> is given as either a <number> or a <percentage>.

If given as a number, the useful range is 0 (fully transparent) to 1.0 (fully opaque), with decimal values in between; that is, 0.5 indicates that half of the foreground color is used and half of the background color is used. Values outside the range of 0 to 1 are permitted, but are clamped to lie within the range 0 to 1.

If the alpha value is given as a percentage, 0% corresponds to fully transparent while 100% indicates fully opaque.

Formal syntax

<alpha-value> = 
<number> |


When animated, values of the <alpha-value> CSS data type are interpolated as real, floating-point numbers. The speed of the interpolation is determined by the easing function associated with the animation.


Setting text color opacity

The rgb() function accepts a fourth optional value to specify an alpha value. The following example shows how to apply a color with 60% opacity using the alpha value:


/* <rgb()> */
color: rgb(34 12 64 / 60%);

Setting shape image threshold

Here an alpha value is used to determine which parts of an image are considered part of a shape:


/* shape-image-threshold */
shape-image-threshold: 70%;
shape-image-threshold: 0.7;


CSS Color Module Level 4
# type-def-alpha-value

See also