Specificity determines which selector takes precedence over another. Let's look at the following code:

<h1 id="main_heading">Heading</h1>

h1 {
	background-color: green;
	color: pink;
}

#main_heading {
	color: yellow;
}

This code would be displayed as follows:

Heading

Why?

Because selectors have the following specificity:

1. id
2. class
3. type
4. universal selector

Therefore, the background color of the heading is green because it is not overridden by a more specific selector.

The color will be yellow because the id selector is more specific than the type selector h1.