Cascading Style Sheets (CSS) basics

Describe the presentation of a document written in HTML or XML (including XHTML, SVG, …)

CSS Basic building blocks

/* multiline-only c-style comments */

CSS Declarations

A declaration assigns a value to a property. The property and the value are separated by the : character.

color: blue

CSS Declarations Blocks

Declarations are grouped in blocks. A block opens with a { and closes with a }. Declarations are separated with a ;.

  color : darkblue ;
  background-color : #EFF4F5

CSS Selectors

Style blocks may apply to a subset of the DOM, not to all the elements of the page. A selector is a condition statement that matches a selection of elements.

#mainmenu, > ul , nav ul

Selection can be based on ids (#myId), classes (.myClass) tags (div), pseudo-classes (:hover), attributes (p[hidden=”true”]), or with general selectors(*[hidden=”true”]).

Search by inheritance is done strictly (direct descendant of) with > or loosely (some descendant of) with a “space”.

CSS Rulesets (or Rules)

A ruleset is the combination of a selector with a block of declarations, where matched elements are applied the declarations in the block.

#mainmenu, > ul , nav ul {
  color : darkblue ;
  background-color : #EFF4F5


Statements that start with an at sign @ followed by an identifier. Then the rest of the statement is up to the next ; (if no block) or the end of a block } (if there is a block). The syntax of the rest may differ for each at-rule.

@charset "UTF-8";  /* Set the encoding of the style sheet to UTF-8 */
@import url("fineprint.css"); /* Import rules from another file */
@font-face {
  font-family: MyHelvetica;
  src: local("Helvetica Neue Bold"),
  font-weight: bold;


The order the navigator chooses to use to select the good rules for the elements.

Order of specificity (by less specific to highly specific):

:warning: the less specific your selectors, the better. Check your selectors’ specificity :

The !important exception

When !important is added at the end of a declaration, this one overwrites all the others made in the CSS for this property with these elements.

The Box Model

Each element is represented as a 4-parts box model.

CSS Box Model @ [MDN](


The position property defines how a block should be positioned in regard to the other blocks in the page with properties such as top, left, right and bottom.

There are 5 possible positioning strategies for the position property.


The default behavior. Blocks are positioned one after the other following the normal flow of the DOM tree.


Position relative to initial position in the flow. When a block is relatively positioned, it’s static position and place used is reserved (a gap is created) and the block is positioned relatively to this initial position (with top, left, right and bottom properties) block still occupies space on the normal flow Leaves a gap at original position.

CSS Relative Positioning from [MDN](


Specify position relative to its closest positioned ancestor or to the containing block. Does not leave space for the element in the normal flow.

CSS Absolute Positioning from [MDN](


Specify position relative to the screen’s viewport. Does not leave space for the element in the normal flow.

CSS Fixed Positioning from [MDN]( CSS Fixed Positioning from [MDN](

sticky CSS 3 Draft 10-2015

Mix between relative et fixed positioning. Behaves like the relative positioning up until it would normally get out of the viewport, then fixed positioning occurs.

:warning: vérifier le support:

CSS Position Demo on CodePen.

Media Queries

Limit the stylesheet’s scope with media types and other properties

@media media-types | media-features {
  /* media-specific rules */

media types CSS 2.1

all, speech, braille, handheld, print, projection, screen, tty, tv, embossed

Media Features CSS 3

height, min-height, max-height, device-width, min-device-width, max-device-width, device-height, min-device-height, max-device-height, aspect-ratio, min-aspect-ratio, max-aspect-ratio, device-aspect-ratio, min-device-aspect-ratio, max-device-aspect-ratio, color, min-color, max-color, color-index, min-color-index, max-color-index, monochrome, min-monochrome, max-monochrome, resolution, min-resolution, max-resolution, scan and grid.

Logical operators CSS 3

and, ,, not and only


@media (max-width: 500px), handheld and (orientation: portrait) {
  aside.adds {
    display: none;
@media only print and (max-device-width:800px) {
  body {

Get Extra Information

The most reliable and up to date source of information about CSS: The Mozilla Developer network.

Check browsers availability for new features on

Vendor Prefixes

Browsers consider some properties with a prefix when standards are not stabilized and the feature’s behavior may change, or when the implementation of a feature is not stabilized in the browser. The code and the syntax may change.

selector {
  -webkit-property : value;
  -moz-property : value;
  -ms-property : value;
  -o-property : value;
  property : value;

Check needed prefixes on ( or use Autoprefixer.

Focus on Some Recent Features

The CSS transition properties

Permit a smooth transition between an old and a new value of a CSS property. CSS properties are changed by:

for(let element of document.getElementsByTagName('a')) {
document.getElementById("d").style.color = "orange";

4 properties:

A transition example

#test_transition:hover {
transition-property: font-size, background-color, color;
transition-duration: 2s, 2s, 10s;
transition-timing-function: ease;
transition-delay: 0s;
font-size: 2em;
background-color: #333;

Transition Demo on CodePen

Short notation may help



transition: font-size 2s ease 0s, background-color 2s ease, color 8s;

The CSS transform property

Move HTML elements on the X and Y axis.

2 properties:

transform : function1(value1)

transform-origin: 50% 50%;
transform-origin: top 0 left 0;

Does transform need vendor prefixes? Not anymore!

#transform_test {
  transform-origin: 50% 50%;
  transform:  scale(1.2)
        translate(100px, 0px);

Simple transform test on CodePen

Mixing transform and transition

Full example on CodePen.

#transform_test:hover {
  transition-property: transform, background-color;
  transition-duration: 2s;
  background-color: red;
  transform: scale(.7) rotate(-1455deg) translate(0px, 0px);

More control with CSS animation

Define property values at key moments of an animation (keyframes). Animations can be repeated.

Several properties:


Full Example on CodePen

#animation_test {
  height: 200px;
  width: 200px;
  border: 1px solid rgba(0,0,0,0.1);
  border-radius: 50%;
#animation_test:hover {
  animation: rainbow 15s 15 linear;
@keyframes rainbow {
  0% {background-color: #FF0000;}
  10% {background-color: #FF8000;}
  20% {background-color: #FFFF00;}
  30% {background-color: #80FF00;}
  40% {background-color: #00FF00;}
  50% {background-color: #00FF80;}
  60% {background-color: #00FFFF;}
  70% {background-color: #0080FF;}
  80% {background-color: #0000FF;}
  90% {background-color: #8000FF;}
  100% {background-color: #FF0080;}

<div id="animation_test" class="third" ></div>

CSS Pre-processors Dynamic CSS

CSS is a limited description language: no function, no inheritance, no arithmetic and no variables until recently.

CSS variables

element {
  --main-bg-color: brown;
element {
  background-color: var(--main-bg-color);

SASS’ Main features (SCSS syntax)


values used in various declarations like themed colors, default values for various sizes (border, padding, margin, spaces, …)

$main-theme-color-bg: #428bca;
  background-color: $main-theme-color-bg;


Inherit properties of a class into another declaration block.

@mixin general_stuff{
  /* ... */
#special_item {
  @include general_stuff;
  /* ... */

Nested Rules

more expressive inheritance and shorter code.

#menu {
  h1 {
    /* ... */
    /* ... */

Mixins with parameters

@mixin big-font ($size: 18px) {
  font-size: $size;
  font-weight: 700;
.important {
  @include big-font;

#more-important {
  @include big-font(25px);

Functions and Operations

Mostly predefined functions. Operations are used within parenthesis.

.something {
  width : ($default-width * 1.3);
  color : darken($default-color, 3%);
  background-color: desaturate($default-color, 10%);