Is it more effective to use id or class ?

Answer 1

depends on what you want.
if your element is the only of such in the dom use id.
if there are multiple similar elements,use class

Answer 2

IDs can only be used once per page so it’s nice if you need to be really specific. If you want to style multiple elements from different webpages, I would use class.

Answer 3

class

Answer 4

dom use Id selector and multiple use class selector