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Styles

Your component's template is rendered to its shadow root. The styles you add to your component are automatically scoped to the shadow root and only affect elements in the component's shadow root.

Shadow DOM provides strong encapsulation for styling. If Lit did not use Shadow DOM, you would have to be extremely careful not to accidentally style elements outside of your component, either ancestors or children of your component. This might involve writing long, cumbersome to use class names. By using Shadow DOM, Lit ensures whatever selector you write only apply to elements in your Lit component's shadow root.

You define scoped styles in the static styles property using the tagged template literal css function. Defining styles this way results in the most optimal performance:

The styles you add to your component are scoped using shadow DOM. For a quick overview, see Shadow DOM.

The value of the static styles property can be:

  • A single tagged template literal.

  • An array of tagged template literals.

The static styles property is almost always the best way to add styles to your component, but there are some use cases you can't handle this way—for example, customizing styles per instance. For alternate ways to add styles, see Defining scoped styles in the template.

Static styles apply to all instances of a component. Any expressions in CSS are evaluated once, then reused for all instances.

For tree-based or per-instance style customization, use CSS custom properties to allow elements to be themed.

To prevent Lit components from evaluating potentially malicious code, the css tag only allows nested expressions that are themselves css tagged strings or numbers.

This restriction exists to protect applications from security vulnerabilities whereby malicious styles, or even malicious code, can be injected from untrusted sources such as URL parameters or database values.

If you must use an expression in a css literal that is not itself a css literal, and you are confident that the expression is from a fully trusted source such as a constant defined in your own code, then you can wrap the expression with the unsafeCSS function:

Only use the unsafeCSS tag with trusted input. Injecting unsanitized CSS is a security risk. For example, malicious CSS can "phone home" by adding an image URL that points to a third-party server.

Inheriting styles from a superclass

Permalink to Inheriting styles from a superclass

Using an array of tagged template literals, a component can inherit the styles from a superclass, and add its own styles:

You can share styles between components by creating a module that exports tagged styles:

Your element can then import the styles and add them to its static styles property:

This section gives a brief overview of shadow DOM styling.

Styles you add to a component can affect:

Lit templates are rendered into a shadow tree by default. Styles scoped to an element's shadow tree don't affect the main document or other shadow trees. Similarly, with the exception of inherited CSS properties, document-level styles don't affect the contents of a shadow tree.

When you use standard CSS selectors, they only match elements in your component's shadow tree. This means you can often use very simple selectors since you don't have to worry about them accidentally styling other parts of the page; for example: input, *, or #my-element.

You can style the component itself using special :host selectors. (The element that owns, or "hosts" a shadow tree is called the host element.)

To create default styles for the host element, use the :host CSS pseudo-class and :host() CSS pseudo-class function.

  • :host selects the host element.
  • :host(selector) selects the host element, but only if the host element matches selector.

Note that the host element can be affected by styles from outside the shadow tree, as well, so you should consider the styles you set in :host and :host() rules as default styles that can be overridden by the user. For example:

Your component may accept children (like a <ul> element can have <li> children). To render children, your template needs to include one or more <slot> elements, as described in Render children with the slot element.

The <slot> element acts as a placeholder in a shadow tree where the host element's children are displayed.

Use the ::slotted() CSS pseudo-element to select children that are included in your template via <slot>s.

  • ::slotted(*) matches all slotted elements.
  • ::slotted(p) matches slotted paragraphs.
  • p ::slotted(*) matches slotted elements where the <slot> is a descendant of a paragraph element.

Note that only direct slotted children can be styled with ::slotted().

Also, children can be styled from outside the shadow tree, so you should regard your ::slotted() styles as default styles that can be overridden.

Limitations in the ShadyCSS polyfill around slotted content. See the ShadyCSS limitations for details on how to use the ::slotted() syntax in a polyfill-friendly way.

We recommend using the static styles property for optimal performance. However, sometimes you may want to define styles in the Lit template. There are two ways to add scoped styles in the template:

Each of these techniques has its own set of advantages and drawbacks.

Typically, styles are placed in the static styles property; however, the element's static styles are evaluated once per class. Sometimes, you might need to customize styles per instance. For this, we recommend using CSS properties to create themable elements. Alternatively, you can also include <style> elements in a Lit template. These are updated per instance.

Limitations in the ShadyCSS polyfill around per instance styling. Per instance styling is not supported using the ShadyCSS polyfill. See the ShadyCSS limitations for details.

Expressions and style elements

Using expressions inside style elements has some important limitations and performance issues.

Limitations in the ShadyCSS polyfill around expressions. Expressions in <style> elements won't update per instance in ShadyCSS, due to limitations of the ShadyCSS polyfill. See the ShadyCSS limitations for more information.

Evaluating an expression inside a <style> element is extremely inefficient. When any text inside a <style> element changes, the browser must re-parse the whole <style> element, resulting in unnecessary work.

To mitigate this cost, separate styles that require per-instance evaluation from those that don't.

While you can include an external style sheet in your template with a <link>, we do not recommend this approach. Instead, styles should be placed in the static styles property.

External stylesheet caveats.

  • The ShadyCSS polyfill doesn't support external style sheets.
  • External styles can cause a flash-of-unstyled-content (FOUC) while they load.
  • The URL in the href attribute is relative to the main document. This is okay if you're building an app and your asset URLs are well-known, but avoid using external style sheets when building a reusable element.

One way to make styles dynamic is to add expressions to the class or style attributes in your template.

Lit offers two directives, classMap and styleMap, to conveniently apply classes and styles in HTML templates.

For more information on these and other directives, see the documentation on built-in directives.

To use styleMap and/or classMap:

  1. Import classMap and/or styleMap:

  2. Use classMap and/or styleMap in your element template:

See classMap and styleMap for more information.

By using CSS inheritance and CSS variables and custom properties together, it's easy to create themable elements. By applying css selectors to customize CSS custom properties, tree-based and per-instance theming is straightforward to apply. Here's an example:

CSS inheritance lets parent and host elements propagate certain CSS properties to their descendants.

Not all CSS properties inherit. Inherited CSS properties include:

  • color
  • font-family and other font-* properties
  • All CSS custom properties (--*)

See CSS Inheritance on MDN for more information.

You can use CSS inheritance to set styles on an ancestor element that are inherited by its descendants:

All CSS custom properties (--custom-property-name) inherit. You can use this to make your component's styles configurable from outside.

The following component sets its background color to a CSS variable. The CSS variable uses the value of --my-background if it's been set by a selector matching an ancestor in the DOM tree, and otherwise defaults to yellow:

Users of this component can set the value of --my-background, using the my-element tag as a CSS selector:

--my-background is configurable per instance of my-element:

See CSS Custom Properties on MDN for more information.