why we should start with id number?

Answer 1

could you precise you question ?

Answer 2

Start what?

Answer 3

when u specify the tags like p,div,h1 or any tags, u specify them with the Id or class,
when u specify class u use .classname same way with id u use #idname, followed by flower brackets n properties of design.

Answer 4

I tend to give the different sections (header, section, aside etc) an id each, and use classes for the various elements embedded within them.

Answer 5

because it works like giving many different ways to single selector

Answer 6

because it's strong than class