Introduction

In the previous lesson, you learned how to write the HTML that determines how a web page is structured. The next step is to make that structure look good with some style, which is exactly what CSS is for. In the next few lessons, we’re going to focus on what we believe are some foundational CSS concepts, things that everyone should know from the beginning — whether they are just starting out or need a refresher.

Lesson overview

This section contains a general overview of topics that you will learn in this lesson.

• Add styles to HTML with CSS.
• Understand how to use the class and ID attributes.
• Add styles to specific elements using the correct selectors.

Basic syntax

At the most basic level, CSS is made up of various rules. These rules are made up of a selector (more on this in a bit) and a semicolon-separated list of declarations, with each of those declarations being made up of a property–value pair.

Selectors

Selectors refer to the HTML elements to which CSS rules apply; they’re what is actually being “selected” for each rule. The following subsections don’t cover every selector available, but they’re by far the most common and the ones you should get comfortable using first.

Universal selector

The universal selector will select elements of any type, hence the name “universal”, and the syntax for it is a simple asterisk. In the example below, every element would have the color: purple; style applied to it.

* {
  color: purple;
}

Type selectors

A type selector (or element selector) will select all elements of the given element type, and the syntax is just the name of the element:

<!-- index.html -->

<div>Hello, World!</div>
<div>Hello again!</div>
<p>Hi...</p>
<div>Okay, bye.</div>

/* styles.css */

div {
  color: white;
}

Here, all three <div> elements would be selected, while the <p> element wouldn’t be.

Class selectors

Class selectors will select all elements with the given class, which is just an attribute you place on an HTML element. Here’s how you add a class to an HTML tag and select it in CSS:

<!-- index.html -->

<div class="alert-text">Please agree to our terms of service.</div>

/* styles.css */

.alert-text {
  color: red;
}

Note the syntax for class selectors: a period immediately followed by the case-sensitive value of the class attribute. Classes aren’t required to be specific to a particular element, so you can use the same class on as many elements as you want.

Another thing you can do with the class attribute is to add multiple classes to a single element as a space-separated list, such as class="alert-text severe-alert". Since whitespace is used to separate class names like this, you should never use spaces for multi-worded names and should use a hyphen instead.

ID selectors

ID selectors are similar to class selectors. They select an element with the given ID, which is another attribute you place on an HTML element. The major difference between classes and IDs is that an element can only have one ID. It cannot be repeated on a single page and should not contain any whitespace:

<!-- index.html -->

<div id="title">My Awesome 90's Page</div>

/* styles.css */

#title {
  background-color: red;
}

For IDs, instead of a period, we use a hashtag immediately followed by the case-sensitive value of the ID attribute. A common pitfall is people overusing the ID attribute when they don’t necessarily need to, and when classes will suffice. While there are cases where using an ID makes sense or is needed, such as taking advantage of specificity or having links redirect to a section on the current page, you should use IDs sparingly (if at all).

The grouping selector

What if we have two groups of elements that share some of their style declarations?

.read {
  color: white;
  background-color: black;
  /* several unique declarations */
}

.unread {
  color: white;
  background-color: black;
  /* several unique declarations */
}

Both our .read and .unread selectors share the color: white; and background-color: black; declarations, but otherwise have several of their own unique declarations. To cut down on the repetition, we can group these two selectors together as a comma-separated list:

.read,
.unread {
  color: white;
  background-color: black;
}

.read {
  /* several unique declarations */
}

.unread {
  /* several unique declarations */
}

Both of the examples above (with and without grouping) will have the same result, but the second example reduces the repetition of declarations and makes it easier to edit either the color or background-color for both classes at once.

Chaining selectors

Another way to use selectors is to chain them as a list without any separation. Let’s say we had the following HTML:

<div>
  <div class="subsection header">Latest Posts</div>
  <p class="subsection preview">This is where a preview for a post might go.</p>
</div>

We have two elements with the subsection class that have some sort of unique styles, but what if we only want to apply a separate rule to the element that also has header as a second class? Well, we could chain both the class selectors together in our CSS like so:

.subsection.header {
  color: red;
}

What .subsection.header does is it selects any element that has both the subsection and header classes. Notice how there isn’t any space between the .subsection and .header class selectors. This syntax basically works for chaining any combination of selectors, except for chaining more than one type selector.

This can also be used to chain a class and an ID, as shown below:

<div>
  <div class="subsection header">Latest Posts</div>
  <p class="subsection" id="preview">
    This is where a preview for a post might go.
  </p>
</div>

You can take the two elements above and combine them with the following:

.subsection.header {
  color: red;
}

.subsection#preview {
  color: blue;
}

In general, you can’t chain more than one type selector since an element can’t be two different types at once. For example, chaining two type selectors like div and p would give us the selector divp, which wouldn’t work since the selector would try to find a literal <divp> element, which doesn’t exist.

Descendant combinator

Combinators allow us to combine multiple selectors differently than either grouping or chaining them, as they show a relationship between the selectors. There are four types of combinators in total, but for right now we’re going to only show you the descendant combinator, which is represented in CSS by a single space between selectors. A descendant combinator will only cause elements that match the last selector to be selected if they also have an ancestor (parent, grandparent, etc.) that matches the previous selector.

So something like .ancestor .child would select an element with the class child if it has an ancestor with the class ancestor. Another way to think of it is that child will only be selected if it is nested inside ancestor, regardless of how deep that nesting is. Take a quick look at the example below and see if you can tell which elements would be selected based on the CSS rule provided:

<!-- index.html -->

<div class="ancestor">
  <!-- A -->
  <div class="contents">
    <!-- B -->
    <div class="contents"><!-- C --></div>
  </div>
</div>

<div class="contents"><!-- D --></div>

/* styles.css */

.ancestor .contents {
  /* some declarations */
}

In the above example, the first two elements with the contents class (B and C) would be selected, but that last element (D) wouldn’t be. Was your guess correct?

There’s really no limit to how many combinators you can add to a rule, so .one .two .three .four would be totally valid. This would just select an element that has a class of four if it has an ancestor with a class of three, and if that ancestor has its own ancestor with a class of two, and so on. You generally want to avoid trying to select elements that need this level of nesting, though, as it can get pretty confusing and long, and it can cause issues when it comes to specificity.

Properties to get started with

There are some CSS properties that you’re going to be using all the time, or at the very least more often than not. We’re going to introduce you to several of these properties, though this is by no means a complete list. Learning the following properties will be enough to help get you started.

Color and background-color

The color property sets an element’s text color, while background-color sets, well, the background color of an element. I guess we’re done here?

Almost. Both of these properties can accept one of several kinds of values. A common one is a keyword, such as an actual color name like red or the transparent keyword. They also accept HEX, RGB, and HSL values, which you may be familiar with if you’ve ever used a photoshop program or a site where you could customize your profile colors.

p {
  /* hex example: */
  color: #1100ff;
}

p {
  /* rgb example: */
  color: rgb(100, 0, 127);
}

p {
  /* hsl example: */
  color: hsl(15, 82%, 56%);
}

Take a quick look at CSS Legal Color Values to see how you can adjust the opacity of these colors by adding an alpha value.

Typography basics and text-align

font-family can be a single value or a comma-separated list of values that determine what font an element uses. Each font will fall into one of two categories, either a “font family name” like "Times New Roman" (we use quotes due to the whitespace between words) or a “generic family name” like serif (generic family names never use quotes).

If a browser cannot find or does not support the first font in a list, it will use the next one, then the next one and so on until it finds a supported and valid font. This is why it’s best practice to include a list of values for this property, starting with the font you want to be used most and ending with a generic font family as a fallback, e.g. font-family: "Times New Roman", serif;

font-size will, as the property name suggests, set the size of the font. When giving a value to this property, the value should not contain any whitespace, e.g. font-size: 22px has no space between “22” and “px”.

font-weight affects the boldness of text, assuming the font supports the specified weight. This value can be a keyword, e.g. font-weight: bold, or a number between 1 and 1000, e.g. font-weight: 700 (the equivalent of bold). Usually, the numeric values will be in increments of 100 up to 900, though this will depend on the font.

text-align will align text horizontally within an element, and you can use the common keywords you may have come across in word processors as the value for this property, e.g. text-align: center.

Image height and width

Images aren’t the only elements that we can adjust the height and width on, but we want to focus on them specifically in this case.

By default, an <img> element’s height and width values will be the same as the actual image file’s height and width. If you wanted to adjust the size of the image without causing it to lose its proportions, you would use a value of “auto” for the height property and adjust the width value:

img {
  height: auto;
  width: 500px;
}

For example, if an image’s original size was 500px height and 1000px width, using the above CSS would result in a height of 250px.

These properties work in conjunction with the height and width attributes in the HTML. It’s best to include both of these properties and the HTML attributes for image elements, even if you don’t plan on adjusting the values from the image file’s original ones. When these values aren’t included, if an image takes longer to load than the rest of the page contents, it won’t take up any space on the page at first but will suddenly cause a drastic shift of the other page contents once it does load in. Explicitly stating a height and width prevents this from happening, as space will be “reserved” on the page and appear blank until the image loads.

Adding CSS to HTML

Now that we’ve learned some basic syntax, you might be wondering how to add all this CSS to our HTML. There are three methods to do so.

External CSS

External CSS is the most common method you will come across, and it involves creating a separate file for the CSS and linking it inside of an HTML’s opening and closing <head> tags with a self-closing <link> element:

<!-- index.html -->

<head>
  <link rel="stylesheet" href="styles.css">
</head>

/* styles.css */

div {
  color: white;
  background-color: black;
}

p {
  color: red;
}

First, we add a self-closing <link> element inside of the opening and closing <head> tags of the HTML file. The href attribute is the location of the CSS file, either an absolute URL or, what you’ll be utilizing, a URL relative to the location of the HTML file. In our example above, we are assuming both files are located in the same directory. The rel attribute is required, and it specifies the relationship between the HTML file and the linked file.

Then inside of the newly created styles.css file, we have the selector (the div and p), followed by a pair of opening and closing curly braces, which create a “declaration block”. Finally, we place any declarations inside of the declaration block. color: white; is one declaration, with color being the property and white being the value, and background-color: black; is another declaration.

A note on file names: styles.css is just what we went with as the file name here. You can name the file whatever you want as long as the file type is .css, though “style” or “styles” is most commonly used.

A couple of the pros to this method are:

1. It keeps our HTML and CSS separated, which results in the HTML file being smaller and making things look cleaner.
2. We only need to edit the CSS in one place, which is especially handy for websites with many pages that all share similar styles.

Internal CSS

Internal CSS (or embedded CSS) involves adding the CSS within the HTML file itself instead of creating a completely separate file. With the internal method, you place all the rules inside of a pair of opening and closing <style> tags, which are then placed inside of the opening and closing <head> tags of your HTML file. Since the styles are being placed directly inside of the <head> tags, we no longer need a <link> element that the external method requires.

Besides these differences, the syntax is exactly the same as the external method (selector, curly braces, declarations):

<head>
  <style>
    div {
      color: white;
      background-color: black;
    }

    p {
      color: red;
    }
  </style>
</head>
<body>
  ...
</body>

This method can be useful for adding unique styles to a single page of a website, but it doesn’t keep things separate like the external method, and depending on how many rules and declarations there are it can cause the HTML file to get pretty big.

Inline CSS

Inline CSS makes it possible to add styles directly to HTML elements, though this method isn’t as recommended:

<body>
  <div style="color: white; background-color: black;">...</div>
</body>

The first thing to note is that we don’t actually use any selectors here, since the styles are being added directly to the opening <div> tag itself. Next, we have the style= attribute, with its value within the pair of quotation marks being the declarations.

If you need to add a unique style for a single element, this method can work just fine. Generally, though, this isn’t exactly a recommended way for adding CSS to HTML for a few reasons:

• It can quickly become pretty messy once you start adding a lot of declarations to a single element, causing your HTML file to become unnecessarily bloated.
• If you want many elements to have the same style, you would have to copy and paste the same style to each individual element, causing lots of unnecessary repetition and more bloat.
• Any inline CSS will override the other two methods, which can cause unexpected results. (While we won’t dive into it here, this can actually be taken advantage of.)

Assignment

Knowledge check

The following questions are an opportunity to reflect on key topics in this lesson. If you can’t answer a question, click on it to review the material, but keep in mind you are not expected to memorize or master this knowledge.

• What is the syntax for class and ID selectors?
• How would you apply a single rule to two different selectors?
• Given an element that has an id of title and a class of primary, how would you use both attributes for a single rule?
• What does the descendant combinator do?
• What are the names of the three ways to add CSS to HTML?
• What are the main differences between the three ways of adding CSS to HTML?

Additional resources

This section contains helpful links to related content. It isn’t required, so consider it supplemental.

• Mozilla CSS values and units can be used to learn the various types of values possible in absolute or relative terms.
• An interactive Scrim which covers much of the material in the lesson in an interactive form.