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Documentation Designing Your Site Components

Lit Web Components

The Web Component spec is an open web standard, an integral part of the fabric of the web and supported by all modern, evergreen browsers. A web component starts life as a custom element in HTML markup which contains one or more dashes, for example <my-button> or <card-heading-title>. You then direct the browser to load that custom element as a dedicated JavaScript object through use of the window.customElements.define method. A web component also comes with a suite of features such as Shadow DOM, Slots, and CSS Shadow Parts to give the component a degree of autonomy and clean separation from the styling and markup concerns of the parent document.

While you can author a web component without using libraries or frameworks of any kind, the lightweight Lit library makes web component development fast & straightforward, offering a fabulous DX (Developer Experience) through features like reactive rendering, attribute reflection as properties (including JSON data!), and template literal syntax.

Through the use of Bridgetown’s Lit Renderer plugin, you can “bake” HTML & CSS from the Lit component into your static or server-rendered output via Declarative Shadow DOM, which is then “re-hydrated” on the client side. You can take advantage of APIs to render up-to-date content in real-time in the browser after possibly-stale static content has first loaded.

Table of Contents #

Installing Lit Renderer #

Simply run the Bundled Configuration automation:

bin/bridgetown configure lit

Or pass it along to the new command:

bridgetown new mysite -t erb -c lit

This will install both the Lit library itself plus the Lit Renderer plugin.

Take Lit for a Spin #

As part of the installation, an example component was provided in src/_components/happy-days.lit.js. It looks like this:

import { LitElement, html, css } from "lit"

export class HappyDaysElement extends LitElement {
  static styles = css`
    :host {
      display: block;
      border: 2px dashed gray;
      padding: 20px;
      max-width: 300px;

  static properties = {
    hello: { type: String }

  render() {
    return html`
      <p>Hello ${this.hello}! ${}</p>

customElements.define("happy-days", HappyDaysElement)

The component establishes some initial styles (:host is the way you apply CSS directly to the custom element itself) and configures a hello reactive property (which gets initialized with whatever is contained within the hello HTML attribute). It then renders out a paragraph tag within its shadow DOM containing the hello text and a current timestamp.

You can use this component in any Ruby template (Liquid not supported). For example, in an .erb template or page:

<%= lit :happy_days, hello: "there" %>

The helper will know how to convert the tag name and attribute keywords to HTML output via Lit’s SSR process. It will look something like this:

  <happy-days defer-hydration hello="there">
    <template shadowroot="open">
        :host {
          display: block;
          border: 2px dashed gray;
          padding: 20px;
          max-width: 300px;
      <p>Hello there! 1654106939801</p>

The <hydrate-root> custom element is provided by Bridgetown’s Lit plugin and establishes the “island” of the the Lit component tree for re-hydration upon page load. Within the <happy-days> component, the <template shadowroot="open"> tag contains the rendered content as part of the declarative shadow DOM spec.

Once you start up your Bridgetown site and visit the page, you should see a box containing “Hello there!” and a timestamp when the page was first rendered.

You can reload the page several times and see that the timestamp doesn’t change, because Lit’s SSR + Hydration support knows not to re-render the component. However, if you change the hello attribute, you’ll get a re-render and thus see a new timestamp. How cool is that?!

Lit Helper Options #

The lit helper works in any Ruby template language and let’s you pass data down to the Lit SSR build process. Any value that’s not already a string will be converted to JSON (via Ruby’s to_json). You can use a symbol or string for the tag name and underscores are automatically converted to dashes.

<%= lit :page_header, title: %>

(Remember, all custom elements always must have at least one dash within the HTML.)

If you pass a block to lit, it will add that additional HTML into the Lit template output:

<%= lit :ui_sidebar do %>
  <h2 slot="title">Nice Sidebar</h2>
<% end %>

You can also pass page/resource front matter and other data along via the data keyword, which then can be used in the block. In addition, if a tag name isn’t present, you can add it yourself in within the block.

<%= lit data: do %>
<% end %>

When the component is hydrated, it will utilize the same data that was passed at build time and avoid a client-side re-render. However, from that point forward you’re free to mutate component attribute/properties to trigger re-renders as normal. Check out Lit’s firstUpdated method as a good place to start.

You also have the option of choosing a different entry point (aka your JS file that contains or imports one or more Lit components). The default is ./config/lit-components-entry.js, but you can specify any other file you wish (the path should be relative to your project root).

<%= lit data:, entry: "./frontend/javascript/components/headers.js" do %>
  <page-header title="${data.title}"></page-header>
<% end %>

This would typically coincide with a strategy of having multiple esbuild entry points, and loading different entry points on different parts of your site. An exercise left for the reader…

Sidecar CSS Files #

The “default” manner in which you author styles in Lit components is to use css tagged template literals (as you saw in the happy-days example above). However, some people prefer authoring styles in dedicated CSS files. The esbuild-plugin-lit-css plugin allows you to author perfectly vanilla CSS files alongside your component files and import them.

One major benefit to this approach is it allows you to process your component CSS through PostCSS using the same configuration and plugins as for other CSS files.

If you’ve updated your site from a version prior to Bridgetown 1.3, you may need to update your esbuild.config.js file so it includes the following configuration option:

  globOptions: {
    excludeFilter: /\.(dsd|lit)\.css$/

In order to separate the “globally-accessible” stylesheets you may have in src/_components from the Lit component-specific stylesheets (which we only want to get instantiated within component shadow roots), we’ll need to use the following file conventions:

  • For global stylesheets, use the standard .css suffix.
  • For Lit component stylesheets, use a .lit.css suffix.

Bridgetown’s bundled Lit configuration provides the building blocks for this setup. You’ll need to edit a few lines in your frontend/javascript/index.js and esbuild.config.js files to opt into this (just look at the comments in the files). Once completed, you’ll be able to write components such as this:

// _src/components/my-nifty-tag.lit.js
import { LitElement, html } from "lit"

import style from "./my-nifty-tag.lit.css" assert { type: "css" }

export class MyNiftyTag extends LitElement {
  static styles = [style]

  // rest of the component definition here

customElements.define("my-nifty-tag", MyNiftyTag)

You can even combine external stylesheets with ones defined directly within a component if you need to share styles between multiple components.

import { LitElement, html } from "lit"

import style from "./shared/components.lit.css" assert { type: "css" }

export class ManyStylesElement extends LitElement {
  static styles = [
      :host {
        border: 1px solid var(--gray-5);
        max-width: 80ch;

  // …

While the esbuild Lit CSS plugin doesn’t require you to include assert { type: "css" } at the end of your import statements, it’s a good idea to get in the habit as it aligns your code with the CSS Module Scripts spec rolling out to browsers now and in future.

In Combination with Ruby Components #

A very powerful pattern for Bridgetown component design is to use a Lit component as the template for a Ruby component. This allows you to use the Ruby component anywhere on your site, along with any pre-processing of data you need it to perform, and then the Ruby component can “emit” a Lit web component upon render. As an example:

class MyRubyComponent < Bridgetown::Component
  def initialize(value:)
    @value = process_value(value)

  def process_value
    @value = "Value: #{@value}"

  def template
    lit :my_lit_component, value: @value
<!-- elsewhere -->
<%= render "Here is my value") %>

In this example, you wouldn’t need a sidecar template for your component in ERB or whatever, because the Lit component serves as the template.

Technical and Performance Considerations #

With a bit of careful planning of which entry point(s) you use, the data you provide, and the structure of your HTML markup within the lit helper, you can achieve good Lit SSR performance while still taking full advantage of the Ruby templates and components you know and love.

More documentation on this is available in the plugin README.

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