Styling Components

Shadow DOM

What is Shadow DOM

Shadow DOM is an API built into the browser that allows for DOM encapsulation and style encapsulation. Shadow DOM shields our component from its surrounding environment. This means that we do not need to be concerned about scoping our CSS correctly, nor worry about our internal DOM being interfered with by anything outside our component.

Browser Support

Shadow DOM is currently natively supported in the following browsers:

  • Chrome
  • Firefox
  • Safari
  • Opera

In browsers which do not support Shadow DOM we fall back to scoped CSS. This gives you the style encapsulation that comes along with Shadow DOM but without loading in a huge Shadow DOM polyfill.

Confused about what scoped CSS is? Don't worry, we will explain this later in detail.

Shadow DOM in Stencil

Shadow DOM is not currently turned on by default for web components built with Stencil. To turn on Shadow DOM in a web component built with Stencil, you can use the shadow param in the component decorator. Below is an example of this:

  tag: 'shadow-component',
  styleUrl: 'shadow-component.css',
  shadow: true
export class ShadowComponent {


Things to remember with Shadow DOM

  • QuerySelector: When using Shadow DOM and you want to query an element inside your web component you must use this.el.shadowRoot.querySelector(). This is because all of your DOM inside your web component is in a shadowRoot that Shadow DOM creates.

  • Global Styles: To externally style a component with Shadow DOM you must use CSS Custom Properties or the proposed CSS Shadow Parts.

  • Normally you would wrap your styles in the tag name of the component like so:

my-element {
  color: black;
my-element div {
  background: blue;

With Shadow DOM enabled, elements within the shadow root are scoped, and styles outside of the component do not apply. As a result, CSS selectors inside the component can be simplified, and the above example could be:

:host {
  color: black;
div {
  background: blue;

Scoped CSS

In browsers that do not currently support Shadow DOM, web components built with Stencil will fall back to using scoped CSS instead of loading a large Shadow DOM polyfill. Scoped CSS automatically scopes CSS to an element by appending each of your styles with a data attribute at run time.

CSS Variables

What are CSS Variables?

CSS Variables are a lot like Sass Variables, but built into the browser. CSS Variables allow you to specify CSS properties that can be used across your app.

Use Case

One use case for CSS Variables is colors. If your app has a primary brand color that is used across your app then instead of writing that same color out each place you need it in your app you can create a variable for it and then use that variable anywhere you need that color in your app. Also, if you ever need to change this color you will only have to change the variable and then it will be updated across your app.

Using CSS Variables in Stencil

Here are the recommended steps to use CSS Variables in Stencil:

  • Create a CSS file to hold your variable definitions. We normally recommend creating a variables.css file in src/global/
  • You can then put this config globalStyle: 'src/global/variables.css' into your stencil.config.ts file.

That's it! Now you can start defining your variables.

Defining CSS Variables

Here is an example of defining a CSS Variable:

/* inside our src/global/variables.css file */

:root {
  --app-primary-color: #488aff;

In this example we have defined a CSS Variable called --app-primary-color that is set to the color #488aff. The :root selector in this example is a CSS pseudo-class that defines the variable on the root element of your project (usually <html>) so that the variable can be used across your app.

Using a CSS Variable

Here is an example of using the CSS Variable that we defined above:

h1 {
  color: var(--app-primary-color)

This will apply the color we defined in our CSS Variable, in this case #488aff, to our h1 element.

IE support

IE11 does not support CSS variables natively, stencil does however provide a best-effort polyfill since it's impossible to polyfill CSS features in the same way JS can be polyfilled.

The stencil polyfill for CSS variables has plenty of limitations with respect a browser supporting it natively, and incurs in heavy performance overhead.

  • Global CSS variables can only be declared in :root or html, they can't be dynamic.

  • Only the stylesheets of shadow or scoped components can have dynamic CSS variables.

  • CSS variables within a component can ONLY be defined within a :host(...) selector.

    :host() {
    /* This works */
    --color: black;
    :host(.white) {
    /* This works */
    --color: white;
    .selector {
    /* This DOES NOT work in IE11 */
    --color: red;
  • CSS variables within a component can be consumed (var(--thing)) in any selector.

The performance overhead of using CSS variables in IE11 is elevated in terms of CPU time and memory. This is because in order to "simulate" the dynamic nature of CSS variables, the polyfill needs to dynamically generate a different stylesheet PER instance. For example, if you have 200 my-cmp elements in the DOM, the polyfill will attach 200 analogous <style> tags to style each element.

The total amount of stylesheets to be handled by IE11 can quickly grow, consuming a lot of memory and requiring a lot of CPU for each Style Invalidation.