Design buck converter to meet output ripple voltage specs
However, how about the total output ripple voltage? Is it equal to sum of ripples caused by individual components?


No, of course not. The output voltage is across the capacitor, so only the capacitor's ripple voltage appears at the output.

The "ripple voltage" across the inductor's series resistance is entirely masked by the much larger voltage being applied to the inductor by the switch. It has no relevance other than to waste power that would otherwise increase the overall efficiency of the converter.


  Another point is that with given output ripple voltage specs, how do you choose inductor and capacitor along with their equivalent series resistance values to meet the output ripple specs?


Choosing parts for a switchmode converter of any type is a multidimensional optimization process involving several interdependent parameters (input to output voltage ratio, switching frequency, transient response, etc.), of which output ripple is just one. There is no simple answer, but it's generally a good idea to pick capacitors with the lowest ESR available.

Sometimes it makes sense to use multiple capacitors in parallel in order to get a better value for the overall ESR. This may also be driven by the need to not exceed the capacitors' ripple current specifications, which is closely related.