Same background colour with various levels of alpha transparency

Regular RGB

RGBA(0)

RGBA(0.5)

RGBA(1)

border-bottom: note that the transparent/translucent colour is overlaid over the background-colour of the element, not the body background. (Should be shades of yellow, not mixed with blue)

black border

RGB(255,255,0) border

RGBA(255,255,0,0) border

RGBA(255,255,0,0.5) border

RGBA(255,255,0,1)

box-shadow does more what you'd want, but is not supported in older browsers IIRC, and possibly more CPU intensive?

black shadow

RGB(255,255,0) shadow

RGBA(255,255,0,0) shadow

RGBA(255,255,0,0.5) shadow

RGBA(255,255,0,1) shadow

border-bottom along with background-clip: padding-box works as you'd hope - but again I suspect support in older browsers is lacking. (Via http://stackoverflow.com/questions/2626039/css-rgba-border-background-alpha-double

black border

RGB(255,255,0) border

RGBA(255,255,0,0) border

RGBA(255,255,0,0.5) border

RGBA(255,255,0,1)