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We can group selectors using a comma (,) separator. The following declaration block will apply to any element that matches either of the selectors in the group:

td, th {
  ⋮ declarations
}

We can think of the comma as a logical OR operator, but it’s important to remember that each selector in a group is autonomous. A common beginner’s mistake is to write groups like this:

#foo td, th {
  ⋮ declarations
}

A beginner might think that the above declaration block will be applied to all td and th elements that are descendants of the element with an ID of "foo". However, the selector group above is actually equivalent to this:

#foo td {
  ⋮ declarations
}
th {
  ⋮ declarations
}

To achieve the true goal, write the selector group as follows:

#foo td, #foo th {
  ⋮ declarations
}