Why is id selector using div and intro resulting to gray backgroundcolor? While class selector used class="first" ?

Question

I'm confused

Answer 1

Can you please link your code?
Without seeing the code, we don't know which ids and classes there are

Answer 2

Answering based on assumption: In CSS, id takes higher precedence than class. Properties written in class for a element is always overwritten by the properties written in a id for the same element. Use !important to give class higher precedence.