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Intro to CSS

CSS is what allows us to make a webpage look the way we want it to.

By linking your HTML page up with a set of CSS rules, you can make your website conform to a more visually appealing standard than black text on white background. You can create large sweeping rules, like making all of the text bright shiny blue and the background a daring shade of magenta, or you can be more surgical and tell every element on the page specifically where to be and how to look.

Often we will use a hybrid approach, telling this section to look like this and this other section over here to have brighter text and a shadowy backdrop.

Learning Objectives

  • Styling with CSS (inline; style tag; *.css)
  • Using the class attribute
  • Using a CSS selector
  • Creating selector based on tag
  • Creating selector based on class
  • Creating selector based on id
  • Creating selector based on location
  • Creating complex selectors
  • Understanding rules of specificity
  • Using multiple CSS classes
  • Adding images to background
  • Using the box model
  • Explaining px vs em vs rem

Getting started with CSS

CSS is a language for specifying how documents are presented to users — how they are styled, laid out, etc.

How does CSS affect HTML?

Web browsers apply CSS rules to a document to affect how they are displayed. A CSS rule is formed from:

  • A set of properties, which have values set to update how the HTML content is displayed, for example, I want my element's width to be 50% of its parent element, and its background to be red.
  • A selector, which selects the element(s) you want to apply the updated property values to. For example, I want to apply my CSS rule to all the paragraphs in my HTML document.

A set of CSS rules contained within a stylesheet determines how a webpage should look

A quick CSS example

The above descriptions may or may not have made sense, so let's make sure things are clear by presenting a quick example. First of all, let's take a simple HTML document, containing an <h1> and a <p> (notice that a stylesheet is applied to the HTML using a <link> element):

The first rule starts with an h1 selector, which means that it will apply its property values to the <h1> element. It contains three properties and their values (each property/value pair is called a declaration):

  • The first one sets the text color to blue.
  • The second sets the background color to yellow.
  • The third one puts a border around the header that is 1 pixel wide, solid (not dotted, or dashed, etc.), and colored black.

The second rule starts with a p selector, which means that it will apply its property values to the <p> element. It contains one declaration, which sets the text color to red.

This isn't too pretty, but at least it starts to give you an idea of how CSS works.

<!DOCTYPE html>
    <meta charset="utf-8" />
    <title>My CSS experiment</title>
    <link rel="stylesheet" href="style.css" />
    <h1>Hello World!</h1>
    <p>This is my first CSS example</p>

h1 {
  color: blue;
  background-color: yellow;
  border: 1px solid black;

p {
  color: red;

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