CSS rule

A typical CSS rule might look something like this:
h1 { font-size: 2em; }
Where:
“h1” is a selector, which defines which part of the HTML to apply the CSS to.
“font-size” is a property, which defines what specific presentational aspect of the
targeted element you want to set.
“2em” is a value, which defines what the property should be set to.
“font-size: 2em;” is known as a statement.
And the whole lot, “h1 { font-size: 2em; }” is collectively known as a rule.