Intro to CSS

Demo starter files

Now that you know the basics of how to structure a web document using HTML, it's time to get started with styling.


Where HTML is meant for the content and logical structure of a web document, CSS (Cascading Style Sheets) handles the visual layout and overall appearance of a page.

The Problem

Before CSS, webmasters used fudgy techniques for spacing and laying out pages. They used blank images to create "spacing", and they used framesets and HTML tables, which were never intended to be used for formatting.

With the introduction of HTML 3.2 in January 1997, new tags like the <font> tag and color attributes made things even worse. Webmasters were now being asked to implement colors and other formatting on web pages, which had to be done tag-by-tag, on every single page. As you can imagine, changing something as simple as a color became a tedious, cumbersome, and expensive task to perform across an entire web site. What a mess!

The Solution

The W3C recognized this problem and was already working on a solution. By December 1997, HTML version 4 was released as a standard, with the first "production ready" draft of CSS along side it. Instead of defining styles and layout in individual tags in the HTML, CSS moved all of the visual settings to an entirely separate file. (Note: To be fair, draft versions of CSS had been in the works since 1994, but the final candidate wasn't released until 1997.)

This solved both problems very effectively. Layout rules are separated from the content, and any number of web pages can reference the same set of rules. Now, when a client requests something like a different font for their heading, the change can be made site-wide in just a single line of code!

CSS has come a long way since then, and new features are being developed and adopted all the time.

An Abstract Medium

Laying out a web page is probably different from almost anything you've encountered before, and it takes some getting used to.

Web sites need to be portable enough to display on myriad device types and display sizes. Unlike print, there is no fixed page size or canvas. You can't double-click on the page to type, or drag objects around to position them on a web page.


So, if a web page doesn't have a fixed size, how do we place things where we want them? We can't really say "2 inches from the top and 3 inches from the left". That certainly doesn't read the same on an iPhone as a computer monitor! There are no truly absolute measurements.

So what's our point of reference? This is partially where the cascade in cascading style sheets comes from.

The only true point of reference we have for elements on a web page are the other elements on the page, relative to each other. If every element demands a certain amount of space for itself, all of the elements can work together to create a flow that pushes things into place.


There's one more thing about cascading that's really important to understand. Elements inherit many properties from their parent. This means that when you apply, say, a font to the <body>, everything down to a list item, within a paragraph, within a div, within the body will inherit that font too.

This is one of the things that makes CSS so powerful. We don't have to set rules for every element. We set general rules on larger elements, and override those rules as we get to smaller, more specific elements and details.

The Box Model

To handle all of this pushing and cascading, style sheets employ a method called the box model.

Every HTML element that renders content on the page is contained in a box. A web page with lots of markup will have a lot of boxes, inside boxes, inside more boxes. This is one of the major reasons why organized, properly nested HTML is so important.

Elements generally fall into one of two categories:

  • Block elements create a space for themselves. They grow as wide as their container so none of their "sibling" elements are allowed to sit next to them, only above or below.

Examples of block elements include <div>, <p>, <h1> through <h6>, <ul>, <ol>, <li>.

  • Inline elements are also contained in a box, but they don't clear a space around themselves, so they can sit next to other elements on the same line.

Examples of inline elements include <a>, <img>, <em>, <strong>, and <span>.

Of course, we can change this behavior in a number of ways when we need to. There are also some gray areas like "inline-block" display, so it's not all black and white!

Here's what the box model looks like for a block level element:

Illustrated CSS Box Model
  • The element content is the actual text or image or media itself.
  • The padding is the space immediately around the content. Other elements will never share this space.
  • The border is the outline, or stroke, around an element.
  • The margin is the space around the outside of the element. Margins "collapse", so adjacent margins can share the same space.

Let's say you have two elements somewhere in your document, one after the other. The bottom margin of one element is 20 pixels, and the top margin of the element underneath it is 10 pixels. Since margins can overlap, the gap between the two elements will be 20 pixels.

Illustrated CSS Margin Overlap

Element 1: "I need at least 20px all around!"
Element 2: "I need at least 10px all around!"

Both elements are satisfied being 20px apart.

Adding CSS to a Document

There are 3 ways to apply styling rules to an HTML document.

  1. Inline, which apply to a single HTML tag, sort of like an attribute
  2. Embedded, or "in-document", which is a style sheet between <style> tags in the <head>
  3. External, using a separate file

We're going to spend most of our time talking about external style sheets, which are the most effective and the most common. It's good to know about the other two methods, but in practice they're rarely used.

The code to link to an external style sheet is as follows, and it goes inside the <head>:


  <link rel="stylesheet" href="style/style.css">

  • The style sheet should go in its own directory, just like images and other types of assets for your web sites.
  • I'm going to name my first style sheet style.css, but you can name it whatever you want, as long as follows normal file naming conventions (lowercase, no spaces, etc.). The file extension must always be .css
  • You can add this <link> to any HTML document whose layout you want to apply from the style.css style sheet. Just make sure the href points to the right place.

You may come across example code online using the <link> tag attribute type="text/css". This is now assumed by modern browsers, so we don't need to use it anymore.

Let's add the <link> tag and fill in the HTML for the demo files, then switch over to the style sheet to follow along.


The syntax, or grammar, of a CSS document is very simple.

There is no doctype declaration in CSS, nor are there any tags like HTML. You should define the character set though:

@charset "utf-8";

UTF-8 is still what we want to use. We talk about other @directives later on, but this is the only one we need to start.

After that, style sheets are basically just a list of rules for the browser to follow when it renders the web page.

h1 {color: red;}

Again, let's break this down:

  • h1 is the selector, or the element in the HTML, that we're targeting.
  • Declarations for that element go between { curly braces }
  • Since we can make more than one declaration at a time, every declaration must end with a ; (semicolon). It's a lot like ending a sentence, telling the browser we're done with this declaration.
  • color is the property we're defining here,
  • and red is the value.

We can make more than one declaration at a time for any given selector:

h1 {color: red; background: lightgray;}

CSS ignores the whitespace between declarations though, just like the space between HTML tags, so you should always put declarations in a readable list form, like this:

h1 {
  color: red;
  background: lightgray;

This will make the <h1> header in our web page appear in red text, on a light gray background.

There are a lot of properties in CSS. The basic ones like color and background we'll use a lot, but there are many others that you probably won't memorize right away, if ever. There are tons of resources online for referencing CSS properties and how to use them. Some text editors even have dictionaries built in.


Comments are just as useful in CSS as they are in HTML. You'll probably use them a lot for toggling rules on and off to see their effect.

In most text editors, the keybinding for toggling a comment is CMD+/ (Command/Control key + Forward Slash). This will comment or uncomment the selected text, or if nothing is highlighted, the line that your cursor is on.

<!-- HTML comment -->

/* CSS Comment */

Style sheets can sometimes grow very long, so comments are also a really helpful way to create visual sections while scrolling through your document.

Selecting Elements

To get you started, we need to cover some basic selectors for targeting elements.

There are many different ways to target elements in HTML. Some you'll use all the time, and some not so much. Here are some of the basic ones, and we'll talk about some of the more advanced selectors in another lesson.

Remember the syntax is:

selector {
  property: value;


Every element of a certain type. This will target all of the <p> tags at once:

  • CSS: p { color: navy; }


A class is like a tag, and you can add them to almost any element in the <body>. Choose a name that implies what it's for, to make it easier to remember. Classes are extremely useful, and you'll use them more than any other kind of selector.

  • HTML: <em class="colored">
  • CSS: .colored { color: navy; }

You can apply a class to just one element, or as many elements as you like. You can also apply multiple classes to the same element, like this:

<a class="colored minor sidebarlink">

This element has three classes: .colored .minor and .sidebarlink.


You can also target elements that you've given an ID to.

  • HTML: <div id="chapter1">
  • CSS: #chapter1 { color: navy; }

Classes and IDs can often be used interchangeably, but since ID's also serve other purposes in HTML, it's almost always better to use classes for styling.


Multiple selectors can be used at once, separated by commas.

  • CSS: h1, h2, h3 { color: navy; }

Basic Colors

There are multiple ways to specify a color in CSS. We'll talk about color in a lot more detail in another lesson. Feel free to look up other methods if you want, but for now the easiest way to specify colors is by choosing from the list of recognized color names.

(Links in the resources below.)

You can either type the name of the color, or use its 6-digit hexadecimal value. If you use the hex value, be sure to precede it with a # sign. Both of these mean the same thing:

.colored { color: olivedrab; }
.colored { color: #6B8E23; }

Olive Drab:

Photoshop's color picker will show the hex value of a selected color. There are also lots of online color pickers via quick google search!

Basic Fonts

Until we talk about typography and fonts in more detail, try to stick to web safe fonts. Web safe fonts are fonts that we can generally assume are installed on any given computer.

(Links in the resources below.)

When defining fonts,

  • Font names are case-sensitive
  • Be sure to quote font names that have spaces
  • You should always provide a generic fallback when you specify a font.
body {
  font-family: Helvetica, sans-serif;
body {
  font-family: "Times New Roman", times, serif;

Basic Units

Like colors, CSS supports several units of measurement for specifying widths and spacing. Feel free to experiment with others, but the easiest ones to work with for now are probably pixels (px) and maybe percentages (%).

  • Pixels are pretty straightforward. They're also a better unit to use than points when specifying font sizes for the screen.
  • Percentages can be confusing for beginners, but they're very important for creating flexible designs. In most cases they're calculated based on the size of the parent element.

Whenever you specify values with units in CSS, make sure there's no space between the number and type of units:

Good: margin: 10px;
Bad: margin: 10 px;

Centering Content

Content on web pages is centered more often than not. There are older methods of accomplishing this that still work, but they are deprecated and shouldn't be used any more. Those methods include <center> tags and the align="center" attribute.

Centering Block Elements

Block elements can have margins. To center block elements, set their left and right margins to auto.

.block-centered {
  margin-left: auto;
  margin-right: auto;

This will center the element, but it might not be immediately obvious. Remember that block elements expand to the full width of their container, so to see the effect of the centering, you'll also have to make their width smaller than their parent.

Centering Inline Elements

Inline content is even simpler to get centered. The text-align property in CSS is used to justify inline elements and text.

.inline-centered {
  text-align: center;

Let's fill in our style sheet to see some of this new stuff in action.