Using the FAST Design System

This section goes into further detail on how the Design System can be used. It will use FAST Frame Design System in the examples but none are specific to FAST Frame; use these features with your own Design System if you're not using FAST Frame.

Setting Design System Properties

Design System properties can always be set by simple property assignment in JavaScript; obtain a reference to the Custom Element instance and assign the property.

Example: Setting a Design System value by property assignment

const provider = document.querySelector('fast-design-system-provider');
provider.designUnit = 6; // Set the Design System property.

or with FAST Element's property bindings:

import { html } from "@microsoft/fast-element";
export const template = html`
<fast-design-system-provider :designUnit="${() => 6}">

Most Design System properties can be set by setting an HTML attribute on the element instance. The attribute name for the Design System property will be the corresponding JavaScript property name by default, but it can be customized or disabled for each Design System property. See Declaring the Design System for more info.

Example: Setting a Design System value by HTML attribute

<fast-design-system-provider design-unit="6">


Every DesignSystemProvider supports the use-defaults boolean attribute. use-defaults is a mechanism to assign the default Design System values to the DesignSystemProvider.

It is a good idea to have a DesignSystemProvider with the use-defaults attribute wrapping all of the app UI to ensure all properties used by descendent components are defined. This is especially important if the Design System properties reflect to CSS custom properties because the CSS custom properties are only created when the value is set.

Example: using use-defaults at the page root

<!DOCTYPE html>
<!-- ... -->
<fast-design-system-provider use-defaults>
<!-- App UI goes here -->

use-defaults can also be used in conjunction with explicit Design System property attribute assignment. If both use-defaults and an attribute setting a Design System property exist, the explicit attribute assignment takes priority.

Example: use-defaults with attribute assignment

<fast-design-system-provider use-defaults design-unit="6">
<!-- the Design System's design unit will be 6, all other Design System values will be initialized to their default -->

Composing Design System Providers

Remember from the overview that the Design System values are inherited and propagate down the DOM tree. This can be used to scope regions of a page with Design System changes.

<fast-design-system-provider design-unit="6">
<fast-design-system-provider design-unit="12">
<p>Design unit is 12px.</p>
<p>Design unit is 6px</p>

CSS Custom Properties

Many Design System properties are intended to be consumed directly in CSS. When a Design System property is configured to emit a CSS custom property, the CSS custom property can be used in CSS for any descendent of the DesignSystemProvider.

Example: Using CSS Custom Properties

p {
margin: calc(var(--design-unit) * 2px) 0;
<fast-design-system-provider design-unit="6">
<p>vertical margin is 12px</p>

CSS Custom Property Registry

There are some things that CSS just can't do: advanced math, complex data structures, and conditionals are simply not possible with CSS today.

To address this, the DesignSystemProvider provides a mechanism to register a JavaScript function that accepts the Design System object and returns a value to be written as a CSS custom property. The registered function will be re-evaluated when the Design System is mutated.

Example: Register a CSS custom property that is a function of the Design System

name: "design-unit-to-the-12th",
value: designSystem => Math.pow(designSystem.designUnit, 12),


The above API is especially useful when defining a component stylesheet; individual stylesheets can declare dependencies on CSS custom properties that are functions of the element instance's Design System through with the CSSCustomPropertyBehavior. This is how the FAST Frame Color Recipes are constructed.

Example: creating a recipe dependency

import { css } from "@microsoft/fast-element";
import { CSSCustomPropertyBehavior } from "@microsoft/fast-foundation";
import { FASTDesignSystemProvider } from "@microsoft/fast-components";
const designUnitToTheTwelfth = new CSSCustomPropertyBehavior(
designSystem => Math.pow(designSystem.designUnit, 12),
const styles = css`
:host {
height: ${designUnitToTheTwelfth.var};