In the example, if I go: #intro em{...} do I get the same outcome of #intro .first em{...}?

Answer 1

Not really. If you want to set styles for all "em" tags in the #intro, use:
#intro em {...}

and if you want to set styles just for first "em" in the #intro, use this:
#intro em:first-child {...}

Answer 2

You're welcome. :)

Answer 3

Thankss