Allows colors to be defined in HSL (Hue, Saturation, and Lightness), HSLA format (Hue, Saturation, Lightness, and Alpha), or RGB plus Alpha format.
HSL, HSLA, and RGBA color values on W3C.
The background color of this paragraph is defined using an RGBA setting of rgba(255, 43, 0, .2)
rgba(255, 43, 0, .2)
Can I Use css3-colors? Data on support for the hsla-rgba feature across the major browsers from caniuse.com.