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.
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%);
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 |