The comma between element selector provides a way to have the same properties to the picked element selectors. Order doesn't matter here
span, strong{
background-color: yellow;
}
For example, this will turn every span and strong with background-color into yellow.
One wrong syntax will ignore the whole element selectors
h1, ..special {
color: blue;
}
# :nth-child():nth-child(-n+3)
Represents the first three elements. [=-0+3, -1+3, -2+3]