CSS Grouping Selectors and Rules Within Same ID or Class

I am not very experienced with CSS, mostly self taught.

I’ve always written CSS like this…

#footer-sections p{font-weight: 400;}
#footer-sections a{font-weight: 700; text-decoration: none;}

Recently I’ve seen others using CSS snippets that are structured like this…

#footer-sections {
p{font-weight: 400;}
a{font-weight: 700; text-decoration: none;}
}

See how the other selectors and rules are combined within the same id, almost like a media query? It seems to work. Is this valid CSS? If so, what is this practice called so I can learn more about it?

Thanks!

• This topic was modified 2 years, 7 months ago by Jan Dembowski. Reason: Moved to Fixing WordPress, this is not an Everything else WordPress topic