CSS Basics

Objectives and Overview

This lesson provides a basic foundation of CSS. This lesson, as well as the remaining lessons in this module, scratch the surface and introduce CSS terminology and syntax.

Lesson Objectives

  • Explain what CSS is and the role it plays in web development.
  • Recognize, understand, and explain common CSS terminology such as selectors, rules, declarations, properties, and values.
  • Create a CSS stylesheet and link to it from an HTML document.

CSS Overview

CSS stands for Cascading Style Sheets and is used to style web content. CSS is a presentation language that affects the appearance of content. If you think of HTML as the body, then CSS can be viewed as the clothes. Fonts, colors, page layout, and certain animations can all be done with CSS as well as much more.

Occasionally you’ll see code where the CSS is mixed in with the HTML. This has its uses, but for this course, you’ll be keeping your HTML and CSS in separate files. You’ll be writing your CSS in an external style sheet. This means that you’ll be writing all of your CSS in a file such as style.css and then linking to that file in your HTML.

The focus of this course isn’t on making you a CSS guru. Rather you should aim to become knowledgeable and familiar enough with CSS to be able to understand how to set and alter the style of your page. There are a wealth of resources available if you’re interested in leveling up your CSS skills.

CSS Term and Syntax

Just like HTML, JavaScript, and any other language, CSS has its own terminology. It’s good to review and use this terminology as much as possible so that you’re able to correctly recognize and understand resources that you discover.

The most important terms that you’ll encounter are selectors, rules, declarations, properties, and values. Let’s look at each of these terms.


Selectors are what you use to select which HTML elements you want to style. You can use combinations of several qualifiers to target unique HTML elements, depending on how specific you want/need to be. For example, you may be content to style all paragraphs on the page the same way, or you may need to be more specific and style only the first paragraph.

This selects all paragraph <p> elements on the page:

p { ...; }

In this example, the p is the selector. Note that when you’re referencing HTML elements in CSS you omit the brackets. For example, you use p instead of <p>.


Once an element or elements is selected, the styles that are applied are set as properties.

There are a large number of CSS properties that you can use, but here are some of the most common: backgroundcolorfont-sizedisplayheight, and width.

The following example sets the color and font-size properties to be applied to all paragraph <p> elements:

p { color: palevioletred; font-size: 12px; }
Code language: CSS (css)

Notice that the property names are contained within brackets { } after a selector, and are separated from their values by a colon.


Values are what dictate the behavior of the property. In the previous example, both “palevioletred” and “12px” are values. Values are the text between the : after the property and before the ; at the end of the line.


declaration is the pair of the property and value/values. Using the previous example, color and font-size are the properties, and palevioletred and 12px are the values. Each of these is a declaration.

Both color: palevioletred and font-size: 12px are declarations.

Here is a helpful infographic from MDN:

CSS Declarations Infographic from MDN

Image Credit: MDN Web Docs


Rules, or rulesets, are the combination of the selector and the declarations. Building on the previous example, the ruleset for p is the entire CSS block:

p { color: palevioletred; font-size: 12px; }
Code language: CSS (css)

Element and Type Selectors

The previous example used an element selector. This is used to target a specific element, such as paragraphs <p>. When working with CSS, you remove the < > signs from the element:

An <h1> element would just be h1 when selecting it in CSS. The <li> element would be li, and so on.

Class and Id Selectors

Another, and much more targeted method for selecting HTML elements, is to use class and id selectors. This only works if the HTML elements have class and id properties.

Once you get in the practice of assigning ids and classes to your HTML elements, using id and class selectors in your CSS will become second nature. The structure is basically the same, as it still requires a selector, property, and value. The only difference is the syntax for selection.

Class Selector Example

For this example, let’s say that you have a <p> with the class “main-content.” The HTML would look like this: <p class=”main-content”>

To select this paragraph element when working with CSS, you use a period and then the classname:

.main-content { color: #2a2a2a; font-size: 12px; font-family: Arial; }
Code language: CSS (css)

Remember, when working with classes you can have multiple elements sharing a classname. You could have multiple <p> elements with the “main-content” class. All of the elements with that class would then use the styles you set.

Id Selector Example

To work with an HTML element that contains a unique id, you use the # and the id name as a selector. For this example, let’s say you have an <h2> with the unique id “special-heading.” The HTML would look like this: <h2 id=”special-heading”>

The style would only impact the <h2> with that particular id. Remember that ids are unique per page, so this is a great way to very specifically target a certain element.

Note on using Id Selectors

While using id selectors is a useful thing to have in your toolbox, it’s considered best practice to avoid using them for CSS purposes. The reasoning for this is rather complex and not necessary to understand right now, but the common best practice for CSS is that you should only use classes.

You may now be wondering when you’d use id selectors. The short answer is that these are often reserved for use with JavaScript interaction with webpages.

The next question you may have is what you’d do if you need to select a specific element with classes. There are many schools of thought on this, but one way is that you can use specific class names that only target a single element and don’t repeat. For now, don’t worry too much about this process and just focus on practicing CSS.

Activity: Create and Link a CSS Stylesheet

While your individual style rules may be different than those of your peers, the process for linking an external stylesheet is the same for everyone.


Before starting, make sure that your stylesheet is in the same folder as your HTML files. These steps assume your stylesheet is in the same folder. It’s much easier (and common) to place the stylesheet in the same folder.

  1. Open one of your projects in VS Code or Glitch
  2. Make sure that you’re in the same folder, and then create a new file
  3. Name this file something like style.css and save it
    • When linking, make sure that you use the actual name of your stylesheet!
    • This example uses style.css for the name.
  4. At the bottom of the <head> section before the </head> tag, add the following:
    • <link rel=”stylesheet” href=”style.css”>
  5. Save your HTML file!
  6. Repeat this process for each HTML file you want to style.

Note: You’ll need to complete this process for every page that you want to use the style rules. For example, if you have an index.htmlabout.html, and a contact.html and intend to use your stylesheet for all of the pages, you’ll need to link to the stylesheet in every HTML file.

Why in the Head?

Notice that the stylesheet link is a similar structure to the anchor (<a>) links you’ve used. This is a similar link structure for when you’ve linked to other pages, except this time you’re linking to a stylesheet instead of another .html file. The rel and type properties are included to clarify that you’re linking to a stylesheet, and then the href is the file name.

The stylesheet is placed in the head (not header!) because you want all of your styles to load before the body. The body contains the content that you’re styling, so you want your style rules to load before the content. If you were to link to the stylesheet after the body, the style rules won’t apply.