What a CSS rule does? How does it looks like?

A CSS rule selects an HTML element and declares how it should look.

A selector identifies the element or elements to be affected by a CSS rule. The declaration is made up of a property and its value, separated by a colon and a space.

selector {
	property: value; /* Declaration */
}

selector {
	property1: value1; /*                   */
	property2: value2; /* Declaration block */
	property3: value3; /*                   */
}