Why give fallback values for box-shadow?

Why give fallback values for box-shadow?
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#1

The following CSS is given in “Applied Visual Design: Add a box-shadow to a Card-like Element”

box-shadow: 0 10px 20px rgba(0,0,0,0.19), 0 6px 6px rgba(0,0,0,0.23);

It looks like the second set of 4 values is given as a fallback in case the first set doesn’t work just like giving backup fonts if the user doesn’t have the first one installed. Why would you need that here though? Why would the first set of values ever not work? Or am I misunderstanding what this does…


#2

That’s not fallbacks; it’s multiple shadows.