CSS intermediate tutorial class and id selector

In the CSS Elementary tutorial, we only consider the HTML selector-in the form of HTML tags.
Of course, you can use the class selector and the identifier selector id to define your own selector.
The benefit of doing so is that depending on the class or id, you can behave the same HTML element differently.
In CSS, the class selector is prior to a half-width English period (.), and the id is prior to the half-width English well number.
It looks like this:

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# Top { Background-color: # ccc; Padding: 1em }  . Intro { Color: red; Font-weight: bold; }

The link between HTML and CSS uses the property id and class. Like this:

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<Div id = "top"> <H1> Chocolate curry <P class = "intro"> This is my recipe for making curry purely with chocolate </p> <P class = "intro"> Mmm mmmmm </p> </Div>

The difference between id and class is that id is used on unique elements, while class is used on more than one element.
You can also apply a selection character to a specified HTML element and place the HTML selection character in front of it. Therefore, p. jam {whatever} will be applied to the section with the class "jam.