Version: 2023.2
Language : English
USS properties
USS common properties

USS data types

USS data types define typical values, including keywords and units, that USS properties and functions accept.


USS properties use the same syntax as W3C CSS documents:

  • Keyword values appear as is. For example: auto, baseline.
  • Basic data types appear between angle brackets ( < and >). For example: <length> and <color>.
  • Non-terminals that share the same name as a property appear between angle brackets and single straight quotes ( <' and '>). For example, <'width'>.

If a property value has more than one option, the following applies:

  • Side-by-side words mean that all options must occur in the given order.
  • A bar (|) separates two or more options. One of them must occur.
  • Double bars (||`) separate two or more options. One or more of them must occur in any order.
  • Double ampersands (&&`) separate two or more options. All options must occur.
  • Square brackets ([ ]) denote grouping.

Every type, keyword, or angle-bracketed group might be followed by modifiers. The following table lists the possible modifiers:

Modifier The preceding type, keyword, or group
An asterisk (*) Occurs zero or more times
A plus sign (+) Occurs one or more times
A question mark (?) Is optional
A pair of numbers in curly braces ({A,B}) Occurs at least A and at most B times


UI Toolkit supports pixelsThe smallest unit in a computer image. Pixel size depends on your screen resolution. Pixel lighting is calculated at every screen pixel. More info
See in Glossary
(px) and percentages (%) as units of measurement for length. Pixel values are absolute, while percentages are relative to the element’s parent.


  • width:200px; expresses a width of 200 pixels.
  • width:50%; expresses a width of half of the parent element’s width.

It’s important to specify the unit of measurement. If you don’t specify a unit of measurement, UI(User Interface) Allows a user to interact with your application. Unity currently supports three UI systems. More info
See in Glossary
Toolkit assumes that the property value is expressed in pixels.

Note: 0 is a special value that doesn’t require a unit of measurement.


Numeric values are expressed as either floating points or integer literals. For example, flex:1.0.


Specific keywords are supported for some built-in properties. Keywords give a descriptive name instead of a number. For example: position:absolute.

All properties support the initial global keyword which resets the default value of a property to an element.

In the following example, although you set the color of all the labels to red, the initial keyword of color restores the color of label1 to its default value:

/* Set the color of all the labels to red. */
Label {
    color: red;
/* label1 is the name of a specific label. */
    color: initial;


UI Toolkit supports the following literal color values and functions:

  • A Hexadecimal value: #FFFF00, #0F0
  • The RGB function: rgb(255, 255, 0)
  • The RGBA function: rgba(255, 255, 0, 1.0)
  • Color keywords


You can reference project assets such as fonts and textures from your USS files. For example, you might reference a texture to use as the background image for an element.

To reference an asset, you can use either the url() function or the resource() function:

  • resource(): Represents an asset in a Resources folder.
  • url(): Represents an asset specified by a path. You can express it as either a relative path or an absolute path.

Use the url() function in most cases. However, the resource() function supports automatically loading different versions of image assets for different screen densities.

Reference assets with the URL function

When you reference an asset with the url() function, the path you specify can be relative or absolute:

  • Relative paths must be relative to the folder that contains the USS file that references the asset.
  • Absolute paths are relative to the project.

The path must include the file extension.

For example, if your project has a USS folder that contains all your style sheets, and a Resources folder that contains all your image assets.

  └─ Editor
      └─ Resources
      └─ USS

To reference an image named thumb.png, you can use one of the following paths:

Relative path Absolute path
url("../Resources/thumb.png") url("/Assets/Editor/Resources/thumb.png")



Reference assets with the resource function

The resource() function can reference assets in Unity’s resource folders (Resources and Editor Default Resources). You reference an asset by name.

  • If the file is in the Editor Default Resources folder, you must include the file extension.
  • If the file is in the Resources folder, you must not include the file extension.

For example:

Path to file Reference syntax
Assets/Resources/Images/my-image.png resource("Images/my-image")
Assets/Editor Default Resources/Images/my-image.png resource("Images/default-image.png")

Reference image assets for High DPI/Retina screens

To support screens with different screen densities (DPI), do the following:

  • Make sure that the high DPI versions of your textures have a @2x suffix in their file names. For example the high DPI version of myimage.png should be myimage@2x.png
  • Place the regular and high DPI versions of the texture assets in the same project folder.

When Unity loads the asset, it automatically chooses the correct version for the current screen DPI.

For example, if you use resource("myimage") in USS, Unity loads either Resources/myimage.png or Resources/myimage@2x.png.


Use quotes to specify a string value. For example: --my-property: "foo".

Additional resources

USS properties
USS common properties