LitElement Styles 2

Information drawn from

LitElement styles

Define scoped styles in the template

We recommend using static styles for optimal performance. However, sometimes you may want to define styles in the LitElement template.

There are two ways to add scoped styles in the template:

Add styles using a <style> element.
Add styles using an external style sheet.

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

In a style element

We recommend using static styles for optimal performance. However, static styles are evaluated once per class. Sometimes, you might need to evaluate styles per instance.

We recommend using CSS properties to create customizable styles. However, you can also include <style> elements in a LitElement template. These are updated per instance.

render() {
  return html`
    <style>
      /* updated per instance */
    </style>
    <div>template content</div>
  `;
}

Expressions and style elements

The most intuitive way to evaluate per-instance styles has some important limitations and performance issues. We consider the example below to be an anti-pattern:

// Anti-pattern!
render() {
  return html`
    <style>
      :host {
        /* Warning: this approach has limitations & performance issues! */
        color: ${myColor}
      } 
    </style>
    <div>template content</div>
  `;
}

Expressions inside a <style> element won’t update per instance in ShadyCSS, due to limitations of the ShadyCSS polyfill. See the ShadyCSS readme for more information.

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

If you need to evaluate expressions inside a <style> element, use the following strategy to avoid creating performance problems:

Separate styles that require per-instance evaluation from those that don’t.
Evaluate per-instance CSS properties by creating an expression that captures that property inside a complete <style> block. Include it in your template.

Example

import { LitElement, html } from 'lit-element';

const perClassStyle = html`
  <style>
    :host {
      display: block;
      font-family: Roboto;
      font-size: 14px; 
    }
  </style>
`;

const blueText = html`
  <style> :host { color: blue; } </style>
`;

const redText = html`
  <style> :host { color: red; } </style>
`;

class MyElement extends LitElement {
  constructor() {
    super();
    this.perInstanceStyle = redText;
  }
  render() {
    return html`
      ${perClassStyle}
      ${this.perInstanceStyle}
      <div>Hello World</div>
    `;
  }
}

customElements.define('my-element', MyElement);

Import an external stylesheet

We recommend placing your styles in a static styles property for optimal performance. However, you can include an external style sheet in your template with a <link>:

import { LitElement, html } from 'lit-element';

class MyElement extends LitElement {
  render() {
    return html`
      <link rel="stylesheet" href="./app-styles.css">
      <button>a button</button>
      <div>a div</div>
    `;
  }
}

customElements.define('my-element', MyElement);

There are some important caveats though:

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.

Dynamic classes and styles

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

The lit-html library offers two directives, classMap and styleMap, to conveniently apply classes and styles in HTML templates.

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

To use styleMap and/or classMap:

Import classMap and/or styleMap:

import { classMap } from 'lit-html/directives/class-map';
import { styleMap } from 'lit-html/directives/style-map';

Use classMap and/or styleMap in your element template:

constructor() {
  super();
  this.classes = { mydiv: true, someclass: true };
  this.styles = { color: 'green', fontFamily: 'Roboto' };
}
render() {
  return html`
    <div class=${classMap(this.classes)} style=${styleMap(this.styles)}>
      Some content
    </div>
  `;
}

classMap syntax

classMap applies a set of classes to an HTML element:

<div class=${classMap({alert:true,info:true})}>Content.</div>
<!-- Equivalent: <div class="alert info">Content.</div> -->

styleMap syntax

styleMap applies a set of CSS rules to an HTML element:

<button style=${styleMap({
  backgroundColor: 'blue',
  border: '1px solid black'
})}>A button</button>

<!-- Equivalent: 
  <button style="
    background-color:blue;
    border: 1px solid black;
  ">A button</button>
-->

To refer to hyphenated properties such as font-family, use the camelCase equivalent (fontFamily) or place the hyphenated property name in quotes (‘font-family’).

To refer to custom CSS properties such as --custom-color, place the whole property name in quotes (‘–custom-color’).

**Inline style or CSS

styleMap equivalent**

background-color: blue;

backgroundColor: ‘blue’ or ‘background-color’: ‘blue’

font-family: Roboto, Arial, sans-serif;	fontFamily: ‘Roboto, Arial, sans-serif’ or ‘font-family’: ‘Roboto, Arial, sans-serif’
`--custom-color: #FFFABC;`	’–custom-color’: ‘#FFFABC;’
`--otherCustomColor: #FFFABC;`	’–otherCustomColor’: ‘#FFFABC;’
`color: var(--customprop, blue);`	color: ‘var(–customprop, blue)’

Examples

Inline style syntax:

<div style="
  background-color:blue;
  font-family:Roboto;
  --custom-color:#e26dd2;
  --otherCustomColor:#77e26d;">
</div>

Equivalent CSS syntax:

div {
  background-color: blue;
  font-family: Roboto;
  --custom-color: #e26dd2;
  --otherCustomColor: #77e26d;
}

Equivalent styleMap syntax:

html`
  <div style=${styleMap({
    'background-color': 'blue',
    fontFamily: 'Roboto',
    '--custom-color': '#e26dd2',
    '--otherCustomColor': '#77e26d'
  })}></div>
`

Theming

Use CSS inheritance to propagate style information to LitElement components and their rendered templates.

<style>
  html {
    --themeColor: #123456;
    font-family: Roboto;
  }
</style>

<!-- host inherits `--themeColor` and `font-family` and
     passes these properties to its rendered template -->
<my-element></my-element>

Use CSS variables and custom properties to configure styles per-instance.

<style>
  html {
    --my-element-background-color: /* some color */;
  }
  .stuff {
    --my-element-background-color: /* some other color */;
  }
</style>

<my-element></my-element>

<my-element class="stuff"></my-element>

// MyElement's static styles
static get styles() {
  return css`
    :host {
      background-color: var(--my-element-background-color);
    }
  `;
}

CSS inheritance

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

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 descendents:

<style>
html { 
  font-family: Roboto;
}
</style>

<div>
  <p>Uses Roboto</p>
</div>

Similarly, host elements pass down inheritable CSS properties to their shadow trees.

You can use the host element’s type selector to style it:

<style>
  my-element { font-family: Roboto; }
</style>

<my-element></my-element>

class MyElement extends LitElement {
  render() { 
    return html`<p>Uses Roboto</p>`; 
  }
}

You can also use the :host CSS pseudo-class to style the host from inside its own template:

static get styles() {
  return css`
    :host {
      font-family: Roboto;
    }
  `;
}
render() {
  return html`
    <p>Uses Roboto</p>
  `;
}

Type selectors have higher specificity than :host.

An element type selector has higher specificity than the :host pseudo-class selector. Styles set for a custom element tag will override styles set with :host and :host():

<style>
  my-element { font-family: Courier; }
</style>

<my-element></my-element>

class MyElement extends LitElement {
  static get styles() { 
    return css`:host { font-family: Roboto; }`
  }
  render() {
    return html`<p>Will use courier</p>`;
  }
}

CSS custom properties

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 available, and otherwise defaults to yellow:

class MyElement extends LitElement {
  static get styles() { 
    return css`
      :host { 
        background-color: var(--my-background, yellow); 
      }
    `;
  }
  render() {
    return html`<p>Hello world</p>`;
  }
}

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

<style>
  my-element {
    --my-background: rgb(67, 156, 144);
  }
</style>

<my-element></my-element>

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

<style>
  my-element {
    --my-background: rgb(67, 156, 144);
  }
  my-element.stuff {
    --my-background: #111111;
  }
</style>
<my-element></my-element>
<my-element class="stuff"></my-element>

If a component user has an existing app theme, they can easily set the host’s configurable properties to use theme properties:

<html>
  <head>
    <title>lit-element code sample</title>
    <script type="module" src="./my-element.js"></script>
    <style>
      html { --themeColor1: rgb(67, 156, 144); }
      my-element {
        --myBackground: var(--themeColor1);
        --myColor: rgb(156, 67, 152);
      }
    </style>
  </head>
  <body>
    <my-element></my-element>
  </body>
</html>

See CSS Custom Properties on MDN for more information.

A simple example theme index.html

<html>
  <head>
    <script type="module" src="./my-element.js"></script>
    <title>lit-element code sample</title>
    <style>
      html {
        --theme-primary: green;
        --theme-secondary: aliceblue;
        --theme-warning: red;
        --theme-font-family: Roboto;
      }
      my-element { 
        --my-element-text-color: var(--theme-primary); 
        --my-element-background-color: var(--theme-secondary); 
        --my-element-font-family: var(--theme-font-family);
      } 
      .warning {
        --my-element-text-color: var(--theme-warning); 
      }
    </style>
  </head>
  <body>
    <my-element></my-element>
    <my-element class="warning"></my-element>
  </body>
</html>

my-element.js

import { LitElement, html, css } from 'lit-element';

class MyElement extends LitElement {
  static get styles() { 
    return css`
      :host { 
        display: block;
        color: var(--my-element-text-color, black); 
        background: var(--my-element-background-color, white);  
        font-family: var(--my-element-font-family, Roboto);
      }
      :host([hidden]) {
        display: none;
      }
    `;
  }
  render() {
    return html`<div>Hello World</div>`;
  }
}
customElements.define('my-element', MyElement);

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Last update on 21 Aug 2020

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