Version: 2020.3
Language : English
Built-in Unity variables
USS properties reference

USS Writing style sheets

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Toolkit has adopted BEM as the methodology for styling elements. While using BEM is not mandatory, it is recommended.

About BEM

BEM stands for Block Element Modifier. BEM is a simple system that helps you write structured, non-ambiguous, easy to maintain selectors. With BEM, you assign classes to elements and then use these classes as the selectors in style sheets.

BEM classes have up to three components:

  • Block name: a block is a meaningful, standalone entity or control. For example, menu, button, list-view
  • Element name: a part of a block that has no standalone meaning and is semantically tied to its block. Element names are appended to the block name using two underscores. For example, menu__item, button__icon, or list-view__item
  • Modifier: a flag that changes the appearance or behavior of a block or element. Append a modifier to a block or element name with two dashes. For example, menu--disabled, menu__item--disabled, button--small, or list-view__item--selected.

Each name part may consist of Latin letters, digits, and dashes. Each name part is joined together with either a double underscore __ or a double dash --.

The following example shows XML code for a menu:

<VisualElement class="menu">
    <Label class="menu__item" text="Banana" />
    <Label class="menu__item" text="Apple" />
    <Label class="menu__item menu__item--disabled" text="Orange" />


Since each element is equipped with classes that describe its role and appearance, you can write most of your selectors with only one class name:

.menu {

.menu__item {

.menu__item--disabled {

You should be able to style elements using a single class name. However, in some cases, you may need to use complex selectors. For example, you could use a complex selector when the style of an element depends on the modifier of its block:

.button {

.button__icon {

.button--small {

.button--small .button__icon {

Be careful that you do not specify long selectors. A long selector could indicate inconsistencies in the graphic design of your UI. You should also avoid using type names (Button, Label) or element names (#my-button) in your BEM selectors.

Making VisualElement BEM-friendly

UI Toolkit adheres to BEM. Each visual element has the necessary class names attached. For example, all TextElement have the unity-text-element class. Each instance of Button, which derives from TextElement, has its class list populated with the unity-button and unity-text-element classes.

If you derive a new element from VisualElement or one of its descendants, following these guidelines to ensure that your element is easy to style using the BEM methodology:

  • Use AddToClassList() in the constructor to add the relevant USS classes to your element instances.
  • If your new element instantiates child elements in its constructor, assign the relevant classes to the children. For example, my-block__first-child, my-block__other-child.
  • If your element supports multiple states or variants, such as small and large, add and remove relevant classes when the element state changes or when the element variant is selected.
  • If your element is meant to be used in other projects, consider prefixing your classes to avoid conflicts with existing user class names.
Built-in Unity variables
USS properties reference
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