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Leveraging CSS

Basic styles#

The final piece of our component story is CSS. Similar to HTML, FASTElement provides a css tagged template helper to allow creating and re-using CSS. Let's add some CSS for our name-tag component.

Example: Adding CSS to a FASTElement

import { html, css, customElement, attr, FASTElement } from "@microsoft/fast-element";
const template = html<NameTag>`  <div class="header">    <slot name="avatar"></slot>    <h3>${x => x.greeting.toUpperCase()}</h3>    <h4>my name is</h4>  </div>
  <div class="body">    <slot></slot>  </div>
  <div class="footer"></div>`;
const styles = css`  :host {    display: inline-block;    contain: content;    color: white;    background: var(--fill-color);    border-radius: var(--border-radius);    min-width: 325px;    text-align: center;    box-shadow: 0 0 calc(var(--depth) * 1px) rgba(0,0,0,.5);  }
  :host([hidden]) {     display: none;  }
  .header {    margin: 16px 0;    position: relative;  }
  h3 {    font-weight: bold;    font-family: 'Source Sans Pro';    letter-spacing: 4px;    font-size: 32px;    margin: 0;    padding: 0;  }
  h4 {    font-family: sans-serif;    font-size: 18px;    margin: 0;    padding: 0;  }
  .body {    background: white;    color: black;    padding: 32px 8px;    font-size: 42px;    font-family: cursive;  }
  .footer {    height: 16px;    background: var(--fill-color);    border-radius: 0 0 var(--border-radius) var(--border-radius);  }`;
@customElement({  name: 'name-tag',  template,  styles})export class NameTag extends FASTElement {  @attr greeting: string = 'Hello';}

Using the css helper, we're able to create ElementStyles. We configure this with the element through the styles option of the decorator. Internally, FASTElement will leverage Constructable Stylesheet Objects and ShadowRoot#adoptedStyleSheets to efficiently re-use CSS across components. This means that even if we have 1k instances of our name-tag component, they will all share a single instance of the associated styles, allowing for reduced memory allocation and improved performance. Because the styles are associated with the ShadowRoot, they will also be encapsulated. This ensures that your styles don't affect other elements and other element styles don't affect your element.


We've used CSS Custom Properties throughout our CSS as well as CSS Calc in order to enable our component to be styled in basic ways by consumers. Additionally, consider adding CSS Shadow Parts to your template, to enable even more powerful customization.

Composing styles#

One of the nice features of ElementStyles is that it can be composed with other styles. Imagine that we had a CSS normalize that we wanted to use in our name-tag component. We could compose that into our styles like this:

Example: Composing CSS Registries

import { normalize } from './normalize';
const styles = css`  ${normalize}  :host {    display: inline-block;    contain: content;    color: white;    background: var(--fill-color);    border-radius: var(--border-radius);    min-width: 325px;    text-align: center;    box-shadow: 0 0 calc(var(--depth) * 1px) rgba(0,0,0,.5);  }

Rather than simply concatenating CSS strings, the css helper understands that normalize is ElementStyles and is able to re-use the same Constructable StyleSheet instance as any other component that uses normalize.


You can also pass a CSS string or a CSSStyleSheet instance directly to the element definition, or even a mixed array of string, CSSStyleSheet, or ElementStyles.

Partial CSS#

There are times when you may want to create reusable blocks of partial CSS, where the abstraction is not valid CSS in and of itself, such as groups of CSS properties or a complex value. To do that, you can use the cssPartial tagged template literal:

import { css, cssPartial } from "@microsoft/fast-element";
const partial = cssPartial`color: red;`;const styles = css`:host{ ${partial} }`;

cssPartial can also compose all structures that css can compose, providing even greater flexibility.


The CSSDirective allows binding behavior to an element via ElementStyles. To create a CSSDirective, import and extend CSSDirective from @microsoft/fast-element:

import { CSSDirective }  from "@microsoft/fast-element"
class RandomWidth extends CSSDirective {}

A CSS directive has two key methods that you can leverage to add dynamic behavior via CSS:


CSSDirective has a createCSS() method that returns a string to be interpolated into an ElementStyles:

class RandomWidth extends CSSDirective {  createCSS() {    return "width: var(--random-width);"  }}


The createBehavior() method can be used to create a Behavior that is bound to the element using the CSSDirective:

class RandomWidth extends CSSDirective {  private property = "--random-width";  createCSS() {    return `width: var(${});`  }
  createBehavior() {    return {      bind(el) {, Math.random() * 100)      }      unbind(el) {;      }    }  }}

Usage in ElementStyles#

The CSSDirective can then be used in an ElementStyles, where the CSS string from createCSS() will be interpolated into the stylesheet, and the behavior returned from createBehavior() will get bound to the element using the stylesheet:

const styles = css`:host {${new RandomWidth()}}`;

Shadow DOM styling#

You may have noticed the :host selector we used in our name-tag styles. This selector allows us to apply styles directly to our custom element. Here are a few things to consider always configuring for your host element:

  • display - By default, the display property of a custom element is inline, so consider whether you want your element's default display behavior to be different.
  • contain - If your element's painting is contained within its bounds, consider setting the CSS contain property to content. The right containment model can positively affect your element's performance. See the MDN docs for more information on the various values of contain and what they do.
  • hidden - In addition to a default display style, add support for hidden so that your default display does not override this state. This can be done with :host([hidden]) { display: none }.

Slotted content#

In addition to providing host styles, you can also provide default styles for content that gets slotted. For example, if we wanted to style all img elements that were slotted into our name-tag, we could do it like this:

Example: Slotted Styles

const styles = css`  ...
  ::slotted(img) {    border-radius: 50%;    height: 64px;    width: 64px;    box-shadow: 0 0 calc(var(--depth) / 2px) rgba(0,0,0,.5);    position: absolute;    left: 16px;    top: -4px;  }

Both slotted and host styles can be overridden by the element user. Think of these as the default styles that you are providing, so that your elements look and function correctly out-of-the-box.

Styles and the element lifecycle#

It is during the connectedCallback phase of the Custom Element lifecycle that FASTElement adds the element's styles. The styles are only added the first time the element is connected.

In most cases, the styles that FASTElement renders are determined by the styles property of the Custom Element's configuration. However, you can also implement a method on your Custom Element class named resolveStyles() that returns an ElementStyles instance. If this method is present, it will be called during connectedCallback to obtain the styles to use. This allows the element author to dynamically select completely different styles based on the state of the element at the time of connection.

In addition to dynamic style selection during the connectedCallback, the $fastController property of FASTElement enables dynamically changing the styles at any time through setting the controller's styles property to any valid styles.

Hiding undefined elements#

Custom Elements that have not been upgraded and don't have styles attached can still be rendered by the browser but they likely do not look how they are supposed to. To avoid a flash of un-styled content (FOUC), visually hide Custom Elements if they have not been defined:

:not(:defined) {  visibility: hidden;}

The consuming application must apply this, as the components themselves do not.