The usage of comma in CSS declarations

Question

What it the logic behind the usage of the comma in CSS declarations?

Sometimes it is required, sometimes it is optional, and sometimes it generates an error.

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Answer 1

brightness and contrast are two separate values of the filter property.

rgba and individual rgb values are parameters of the rgba and linear-gradient functions, in this case linear-gradient is the value of the background property.

Edit: also worth noting there is a major exception to this rule: font-family takes several values separated by commas... So no this isn't entirely consistent.