A CSS stylesheet consists of a set of CSS rules that applies to the corresponding elements such as heading, paragraphs, lists, tables, images, etc. in the HTML document.
A CSS rule consists of two main parts, first – a selector and second – a declaration block.

Selector: A selector is the HTML element that you want to style. For example – <h1>, <h2>, <p>, <span>, etc.
Declaration Block: The declaration block contains one or more declarations separated by semicolons(;
). Each declaration consists of a CSS property and a value separated by a colon(:
). For example, color: red;
, background: yellow;
, etc. Each declaration block is surrounded by curly braces {}
.
A property is the style attribute that you want to change, for example – color
, background-color
, font-size
, etc. Each property has a value, for example, the background-color
could be red, green, yellow, and so on.
Example:
h1{ color: red; background-color: yellow; }
Explanation:
In the above example, h1
is a selector(The HTML element that we want to style).
The color
and the background-color
are CSS properties, red
and yellow
are their corresponding values.