CSS
CSS, Cascading Style Sheets - is not a programming language, it is the code that styles web content (of the website built in a markup language such as HTML). In HTML every element on screen can be considered as a box. And box model is an essential term in CSS.
Each box has 4 areas: centrally located is content area (e.g. block of text, a table or an image) and around are padding area, border area and margin area.
![css5 css5](http://docs-1-4.wizlink.eu/wp-content/uploads/2021/06/css5.jpg)
![css5-2 css5-2](http://docs-1-4.wizlink.eu/wp-content/uploads/2021/08/css5-2.jpg)
The source of both illustrations is www.kew.org. In MS Edge, Chrome and Opera you can find the left one; the right one is taken from Firefox. They differ only in colors (not in functions).
Each area is defined by a quite vast list of properties. CSS describes each element of the box giving it features such as color, size, positioning, relationships between elements in the document tree and so on. A set of such features is called stylesheet. From the programmer's point of view, a stylesheet consists of a list of rules. Each rule consists of two main parts: selector and a declaration block written inside curly braces (which has two parts: property name, followed by a colon (:), followed by a property value).
This is an example of valid code:
h1
{
background-color: black;
color: lime
}
It means: each first-level headers - h1 - in a document onscreen has black background with light green text (other features, like letters weight, are set in other rule).
In short: a CSS selector indicates (“selects”) the HTML element(s) you want to style, a declaration part defines its properties and gives them a specific value.