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Originally Posted by AlanHK

But what does omitting the comma do?

Think of it as a descendant selector.

div p img { ... }

will match all img tags that are inside of p tags that are inside of div tags. Although the p could be inside of something else that's inside the div, likewise for the img; it's a descendant selector meaning the p could be a child, grandchild, great grandchild, etc. For immediate descendant (child) you'd use >;

div > p > img { ... }