Lesson Overview

Note that you might want to bookmark this page to use as a reference. You don't necessarily need to go through it all now: you might prefer to learn more about selectors or properties, first.

Already in the two previous lessons you probably started learning how to use some CSS properties like background-color, color, border, font-weight, margin, and a few others. We'll take time to look at these and other properties in later tutorials. You probably also noticed the different kinds of values assigned to the various properties you were exposed to. This tutorial goes over some of the more common values and will give you some references where you can learn more. You'll also learn more about certain kinds of values when you learn about the properties that use those values.


Make sure that you've worked through the previous tutorial, CSS Syntax before proceeding through this lesson.

Types Summary

Many CSS properties can accept different types of data, and some CSS properties will only accept one type of data. This section summarizes some of the more common types of data you'll encounter when assigning values to CSS properties. you can find more details and additional types of values in the MDN CSS Values and Units Reference.

CSS-Wide Values

There are a few values that are accepted by all properties:

Used to set a property back to its default value.
Using the declaration all: initial; will set all properties back to their default values.

Here's an example on CodePen:

See the Pen CSS initial property by Wendi Jollymore (@ProfWendi) on CodePen.

(direct link to the initial value demonstration CodePen, opens in the pen tab)

Used to set a property to its inherited value. If an element is a child of another element, the child element's properties inherit (or copy) their values from the parent element.
You'll usually use this value when you're trying to override a previous rule, such as in this example:

See the Pen CSS Inherit Value by Wendi Jollymore (@ProfWendi) on CodePen.

(direct link to the inherit value demonstration CodePen, opens in the pen tab)

Used to reset a property back to either it's inherited value or its initial/default value.
If the property belongs to a child element, it acts like inherit, otherwise it acts like initial.


Many CSS properties accept a value with a specific unit of measurement. For example, properties that measure the length or size of an object take a length. Length is a type of dimension. A dimension is any valid number value followed by a unit of measurement.

There are several units of measure that are permitted with CSS dimensions, too many to get into in this course, but here are the most common you'll use:

Absolute Units

Absolute units are not used as much in CSS as they used to be, as they don't promote responsive design. Responsive web pages adjust or respond to changes in the device screen orientation, resolution, system font size, etc. Using absolute units sets a fixed size for an element that doesn't change, even when the screen is resized.

pixels (1/96th of an inch)
Note that for advanced CSS, it will be important to understand the difference between device pixels and CSS pixels.
Pixels are often used to measure height and width of elements, or font size. Before responsive and accessible design was considered important, pixels were also often used to define how much margin/padding an element add, and how thick the element's borders were. In modern design, this has been replaced with relative units.
points, which are often used to measure typefaces and fonts (1/72nd of an inch)
the pt unit is used to set the font size for printed documents and should never be used for screens (for reference, 16px = 12pt).

There are additional units such as cm (centimetres) and in (inches) but these are rarely used in modern web design. For more information, see MDN: CSS Values and Units (opens in ref tab).

Relative Units

Relative units are the preference for modern, responsive, and accessible web design. Relative units measure "in relation to" something of a set or fixed size. For example, you can set the font size to be 1.5 times the size of the document's default font size, or you can set an image to be 50% of the screen's width. Relative units allow items to scale and adjust to changes in orientation, screen size, screen resolution, and the user's system font settings.

pronounced "em"; it stands for the letter "M": in typography, the letter M was used to identify the standard height and width measurement for a specific font (source: W3C: The amazing em unit and other best practices)
refers to a measure in relation to an element's font or the element's parent's font
e.g. if the document is set to have a default font size of 20px, you can set an element in the document to have a font size of 1.0em (20px), .5em (10px), 1.5em (30px), etc.
although em is a measure in relation to the font size, it is also often used to measure an element's margin, padding, and border width
elements inherit property values from their parent elements, therefore, be wary when using ems on child elements: see this codepen for a demonstration Font-Size: Cascade and EMs
stans for "root em"
works just like em, except that it always refers to the font size of the "root element"
"root element" generally refers to the <html> element
you can use the html selector to style the root element, or you can use the :root selector (the difference is discussed in an upcoming lesson, link TBD)
more information on the :root selector: Pseudo Classes: the :root pseudo class
vw and vh
vw = "viewport width"; vh = "viewport height"
The "viewport" refers to the visible area of the browser window, so it doesn't include the browser's borders, menu bar, title bar, etc.
1vw = 1% of the viewport width, 1vh = 1% of the viewport height (for example, if your screen is 1920px wide by 1080px high, 10vw = 192px and 10vh = 108px; 50vw/50vh is 50% of the viewport width/height; 100vw/100vh can be used to set something to be equal to the viewport width/height)
these units are often used to set the size of document sectioning elements or other containers, such as menus, article/section elements, divs and other containers, etc; it's also useful when you need to set the size of an element to be the same width and/or height of the screen
simiarly, vmin can be used instead of vw/vh to use the smaller value of a viewport's width/height: for example, 50vmin refers to 50% of the viewport's height or width, whichever is smaller
simiarly, vmax can be used instead of vw/vh to use the larger value of a viewport's width/height: for example, 50vmax refers to 50% of the viewport's height or width, whichever is larger
% (percentages)
used to indicate a percentage of the element's parent container size.
for example, width: 50%; means the element's width is set to 50% of its container's width
when used with the font-size property, it refers to a percentage of the parent element's font-size value (e.g. font-size: 10%; is the same as font-size: .1em;

There are additional units such as lh (line height) and rlh (root line height). For more information, see MDN: CSS Values and Units (opens in ref tab).

Colour Values

Many CSS properties accept a colour value, such as color (the element's text colour), background-color (the element's background colour), and border-color (the colour of the element's border, if it has one). There are several ways to specify the colour value, but here I will only talk about the most common ones. See MDN: CSS Values and Units - Color (opens in the ref tab) and MDN: CSS color type values (opens in the ref tab) for more information.

CSS Named Colours

There are about 166 pre-defined keywords for various colour values. You can use any of these to set a property to a specific colour. You can see the full list in the MDN documentation for the named colour type.

For example, I could set the foreground and background colors of an <aside> element like this:

aside {
  background-color: gray;
  color: navy;

You'll notice that named colours are in all lower-case, although most browsers support mixed case. Regardless, the standard is to use all lower-case for colour names.

In addition to these named colours, you can also use either of the following values:

refers to the element's current color value
generally you would use this to set the value of of an element's property that doesn't already receive a default colour
see also MDN: currentcolor keyword (opens in the ref tab)
refers to full transparency (or alpha channel, if that term is familiar to you); a transparent item is "see-through", like a glass window (the opposite is "opaque" (sounds like oh-PAKE) - a brick wall is opaque, you can't see through it)
see also MDN: transparent keyword (opens in the ref tab)

Hexadecimal Colour Values

If the colour you wish to use is not one of the named colours available in CSS, you can be more specific by using a hexadecimal value. To specify the hex colour value, you must use the following syntax:




Note that #RGB is only used when the digits/letters for RR, GG, and BB are the same. For example, #33FF66 is equal to #3F6.

The # (number sign or hash-tag symbol) indicates that the value is a hexadecimal colour value. The letters following the # sign are as follows:

RR or R
Red channel: A hexadecimal value from 0 to FF for the amount of red.
GG or G
Green Channel: A hexadecimal value from 0 to FF for the amount of green.
BB or B
Blue Channel: A hexadecimal value from 0 to FF for the amount of blue.

Typically you would use a colour picker or mixer to obtain a hex colour value. I like these ones, but there are many others available on the world wide web:

For example, the code #33FF66 is a sort of lime green:

aside {
  border-color: #33FF66;

Note that you could have also said:

aside {
  border-color: #3F6;

In CodePen.io, you can also use the built-in colour picker: just click on "Assets" on the bottom-left of the screen, then choose "Colors". You can then click on a colour chip, and the hex code will be copied to the clipboard. Close the Assets/Color dialog to go back to your code, where you can paste the colour code wherever you want to use it.

the codepen colour picker
The CodePen.io Colour Picker

The rgb()/rgba() Functions

Another common way to refer to a specific colour that isn't already defined with one of the CSS named colour values is using either the rgb() or rgba() function.

The rgb() function accepts three integer values, each from 0 to 255, for the value of each colour channel: The first argument is the red channel, the second value is the green channel, and the third value is the blue channel. For example, earlier I used the colour code #33FF66 or #3F6. The hex value 33 has an integer value of 51, the hex value FF has a value of 255, and the hex value 66 has a value of 102. Therefore, I could have created the same CSS by using the rgb() function as follows:

aside {
    border-color: rgb(51, 255, 102);

As with hex codes, you would likely use a colour picker or colour mixer to determine the integer values to use for a specific colour.

The rgba() function adds a fourth argument for the alpha channel. The alpha channel defines how much transparency or opacity should be included: a value of 0 means fully transparent and a value of 1.0 means fully opaque. Anything in between means partially transparent (e.g. .5 would be 50% transparent). Generally you would use transparency on the colour of an element that is over top of another, otherwise the transparency just has the appearance of making the colour less intense.

You can see an demonstration of the rgba() function in this CodePen:

See the Pen CSS: Colours - Alpha by Wendi Jollymore (@ProfWendi) on CodePen.

(direct link to the rgba() demo CodePen, opens in the pen tab)


Fork/copy the following CodePen and play around with the various colour values: make sure you review the HTML and also read the comments in the CSS.

See the Pen CSS Colour Values by Wendi Jollymore (@ProfWendi) on CodePen.

(direct link to the colour exercise CodePen, opens in the pen tab)