Measurements, URLs, and Color Units - The CSS - page 65

Figure 4.1
Using a transpar-
ent background
color to make
solid text more
readable against
a background
image.
rgba(r,g,b,a)
The r , g , and b values work on the same scale as their rgb() unit coun-
terparts discussed earlier in this chapter. The alpha ( a ) value is a decimal
number from 0 to 1, with 0 being completely transparent and 1 being
fully opaque.
section { background-color: rgba(0,200,0,0.1) ; } /* very
transparent green */
Color units with alpha values of 1 are equivalent to using the solid
color unit. Consider using that solid color unit instead because it may
be supported by more browsers.
hsla(h,s,l,a)
Like rgba() , hsla() is the same color as hsl() with an added designation
for alpha transparency. This unit is also a decimal number from 0 to 1.
a { background-color: rgba(0,0,0,0.9) ; } /* almost solid
black */