Note: This document is a work in progress. You can help improve it.

CSS Declarations

Setting CSS properties to specific values is the core function of the CSS language. What is important to remember is that both properties and values are case-sensitive in CSS and that the property and value in each pair is separated by a colon (:).

There are more than 300 different properties in CSS and nearly an infinite number of different values. Not all pairs of properties and values are allowed; each property has a specific list of valid values defined for it.

We have listed many of our favorite CSS properties that we feel represent the most typical properties a developer should be familiar with.

CSS declaration blocks

Declarations are grouped in blocks, with each set of declarations being wrapped by an opening curly brace, { and a closing one }.

Each declaration contained inside a declaration block has to be separated by a semi-colon (;), otherwise the code won't work (or will at least give unexpected results.) The last declaration of a block doesn't need to be terminated by a semi-colon, though it is often considered good style to do so as it prevents forgetting to add it when extending the block with another declaration.

NOTE: We highly recommend using the tool Prettier(https://prettier.io) to format your HTML, CSS, and JavaScript as it will apply good code style as you are learning. The developer tool setup instructions we supply document how to enable this.

CSS selectors and rules

We are missing one part of the puzzle — we need to discuss how to tell our declaration blocks which elements they should be applied to. This is done by prefixing each declaration block with a selector — a pattern that matches some elements on the page. The associated declarations will be applied to those elements only. The selector plus the declaration block is called a ruleset, or often simply just a rule.

Selectors can get very complicated — you can make a rule match multiple elements by including multiple selectors separated by commas (a group) and selectors can be chained together. For example I want to select any element with a class of blah, but only if it is inside an <article> element, and only while it is being hovered by the mouse pointer. Don't worry — things will become clearer as you become more experienced with CSS, and we'll explain selectors in great detail.

Shorthand

Some properties like font, background, padding, border, and margin are called shorthand properties — this is because they allow you to set several property values in a single line, saving time and making your code neater in the process.

For example, this line:

/* in shorthand like padding and margin, the values are applied
in the order top, right, bottom, left (the same order as an analog clock, clock-wise). There are also other
shorthand types, for example two values, which set for example
the padding for top/bottom, then left/right */
padding: 10px 15px 15px 5px;

Does the same thing as all these:

padding-top: 10px;
padding-right: 15px;
padding-bottom: 15px;
padding-left: 5px;

Whereas this line:

background: red url(bg-graphic.png) 10px 10px repeat-x fixed;

Does the same thing as all these:

background-color: red;
background-image: url(bg-graphic.png);
background-position: 10px 10px;
background-repeat: repeat-x;
background-scroll: fixed;

We won't attempt to teach these exhaustively — you'll come across many examples as you work with CSS.