Web Components

Build or maintain a web component application using $ARGUMENTS. Apply all rules below strictly.

Guiding Principles

• web standards and web platform first
• minimal external dependencies — every dependency must justify its existence, and is continuously replaced with web standards
• no frameworks — use the platform: custom elements, ES modules, import maps
• no build system — dependencies are vendored as self-contained ES modules; nothing to compile, bundle, or transpile
• progressive enhancement over JavaScript-first design

Composition

Compose with /web-conventions — it provides the baseline rules for semantic HTML, accessibility, modern CSS, design tokens, and the Baseline browser-support policy — and with /javascript-conventions, which provides the language-level JavaScript rules (modules, syntax, functions, collections, asynchrony, errors, JSDoc types) and the Baseline lookup for JavaScript features. Rules in this skill override both (e.g. container-query-first responsiveness). For static content sites without client-side state, use /web-static instead; for a static site needing only a few isolated enhancements, /web-sprinkles. For experiments and PoCs, /web-latest may be composed on top to lift the Baseline browser-support constraints.

Reference Implementation

This skill is based on the bce.design quickstarter. The naming rules below are stricter than the single-module quickstarter (which registers unprefixed tags like b-add) — multi-module applications need module-prefixed tag names.

Allowed Dependencies

Only one runtime dependency is permitted. Do not add others without explicit approval:

1. lit-html — templating and efficient DOM rendering

State management needs no dependency — reduction.js, a minimal in-app store (~70 lines), implements the used Redux Toolkit API (configureStore, createAction, createReducer) on top of the structuredClone() web standard (see State Management below). The original Redux Toolkit may be vendored as an optional, import-map-switchable alternative.

Client-side routing needs no dependency — it is implemented with web standards (Navigation API + URLPattern, both Baseline Newly Available; see Routing below).

Development tooling: any static web server with an index.html fallback (e.g. zws, bundled with web-conventions, where --single provides the fallback and --live adds reload-on-save while authoring) and Playwright (E2E tests). There is no build system.

BCE Architecture (Boundary Control Entity)

Structure code using the Boundary Control Entity pattern. Organize by feature module, not by technical layer:

app/src/
├── BElement.js              # base class for all custom elements
├── store.js                 # store configuration
├── reduction.js             # minimal standards-based store (Redux Toolkit API subset)
├── router.js                # standards-based client-side router (Navigation API + URLPattern)
├── app.js                   # entry point, route configuration, persistence
├── app.config.js            # application metadata (name, version)
├── index.html               # single HTML entry point
├── tokens.css               # design-token custom properties
├── style.css                # application styles
├── libs/                    # vendored third-party ESM modules (lit-html)
├── [feature-name]/          # one folder per feature module (business component)
│   ├── package-info.md      # the module's responsibility & design decisions
│   ├── boundary/            # UI web components
│   ├── control/             # actions and dispatchers
│   └── entity/              # reducers and state shape
└── localstorage/
    └── control/
        └── StorageControl.js  # persistence layer

DESIGN.md and the W3C Design Tokens file tokens.json live at the repository root (see /web-conventions); tokens.css is their CSS custom-property projection.

Boundary (UI Components)

• all components extend BElement — never extend HTMLElement directly
• components are the only layer that imports html from lit-html
• components dispatch actions through control functions — never dispatch directly to the store
• override view() to return a lit-html template — this is the only required method
• override extractState(reduxState) to select the relevant state slice
• access extracted state via this.state inside view()
• register every component with customElements.define('prefix-name', ClassName)
• use b- prefix for component tag names
• composite components import and compose child components
• each module's boundary exposes one primary entry point named after the module — bookmarks/boundary/Bookmarks.js registering b-bookmarks — the module's default routing target
• additional routed views and child components carry the module prefix in their tag name (b-bookmarks-add) — tag names are global, so unprefixed names collide across modules

import BElement from "../../BElement.js";
import { html } from "lit-html";
import { deleteItem } from "../control/CRUDControl.js";

class ItemList extends BElement {
    extractState({ items: { list } }) {
        return list;
    }
    view() {
        return html`
        <ol>
            ${this.state.map(item => html`
                <li>${item.label} <button @click="${_ => deleteItem(item.id)}">delete</button></li>
            `)}
        </ol>
        `;
    }
}
customElements.define('b-items-list', ItemList);

Control (Actions & Dispatchers)

• define actions with createAction imported from @reduxjs/toolkit — the import map resolves it to reduction.js
• export both the action creator and a dispatcher function for each action
• dispatcher functions encapsulate store.dispatch() — boundary components call dispatchers, never dispatch directly
• use Date.now() for generating entity IDs on the client

import { createAction } from "@reduxjs/toolkit";
import store from "../../store.js";

export const itemUpdatedAction = createAction("itemUpdatedAction");
export const itemUpdated = (name, value) => {
    store.dispatch(itemUpdatedAction({ name, value }));
}

export const newItemAction = createAction("newItemAction");
export const newItem = _ => {
    store.dispatch(newItemAction(Date.now()));
}

Entity (Reducers & State)

• use createReducer with builder callback (imported from @reduxjs/toolkit, resolved to reduction.js)
• case handlers receive a structuredClone of the current state — mutate it freely or return a replacement; there is no Immer
• define initialState as a plain object with sensible defaults
• use a temporal cache object pattern for form input before committing to a list
• keep state shape flat — avoid deep nesting

import { createReducer } from "@reduxjs/toolkit";
import { itemUpdatedAction, newItemAction } from "../control/CRUDControl.js";

const initialState = {
    list: [],
    item: {}
}

export const items = createReducer(initialState, (builder) => {
    builder.addCase(itemUpdatedAction, (state, { payload: { name, value } }) => {
        state.item[name] = value;
    }).addCase(newItemAction, (state, { payload }) => {
        state.item["id"] = payload;
        state.list = state.list.concat(state.item);
    });
});

Documentation

package-info.md is the web counterpart of Java's package-info.java — a Markdown file at the feature-module root (app/src/[feature-name]/package-info.md) that documents the business component at the folder level, co-located with its code.

• at minimum, state the module's single responsibility and the design decisions behind it — not its contents or a file listing
• keep it focused; do not restate the BCE pattern itself or explain boundary/control/entity generically

When the project is driven by SBCE (/sbce), this file is the capability spec — it holds the BC's full boundary contract (responsibility, ## Boundary operations, ## Requirements as EARS statements, optional ## Entities, ## Out of scope). One package-info.md per business component; it is the single source of truth that /sbce apply converges the code to.

JSDoc

Plain ES modules carry no type annotations — JSDoc provides them, per /javascript-conventions. In this stack, the @typedef for an entity and its state shape lives in the entity layer of the owning module; boundary and control reference it rather than redeclaring it.

BElement Base Class

The base class provides automatic store integration for all components:

• connectedCallback() — subscribes to the store, triggers initial render
• disconnectedCallback() — unsubscribes to prevent memory leaks
• triggerViewUpdate() — extracts state, generates template, renders via lit-html
• view() — abstract, must be overridden to return a lit-html template
• extractState(reduxState) — override to select a state slice (default: entire state)
• getRenderTarget() — override to change render target (default: this)

Do not modify the BElement base class unless adding cross-cutting concerns that affect all components.

State Management

reduction.js — a minimal redux

State management is unidirectional Redux-style data flow without the Redux dependency. reduction.js implements the used Redux Toolkit API surface — configureStore, createAction, createReducer — in ~70 lines of standards-based code. Immutability comes from structuredClone() instead of Immer: each case handler receives a clone of the current state and may mutate it freely, or return a replacement state.

The implementation is selected in the import map (index.html): @reduxjs/toolkit resolves to /reduction.js by default. The original Redux Toolkit, vendored in libs/, remains a one-line import-map switch away — application code is identical for both.

Store Configuration

import { configureStore } from "@reduxjs/toolkit";
import { load } from "./localstorage/control/StorageControl.js";
import { items } from "./items/entity/ItemsReducer.js";

const reducer = { items }
const preloadedState = load();
const config = preloadedState ? { reducer, preloadedState } : { reducer };
const store = configureStore(config);
export default store;

localStorage Persistence

• persist the entire application state to localStorage on every store update via store.subscribe()
• use JSON.stringify / JSON.parse for serialization
• derive storage key from appName in app.config.js
• wrap save/load in try-catch — never let persistence errors crash the application
• preload persisted state on store initialization

Data Flow

User Event → Boundary Component → Control (dispatcher) → Action
→ Entity (reducer) → Store → BElement.triggerViewUpdate()
→ extractState() → view() → lit-html render() → DOM

Routing

Routing is implemented with web standards — no router library. URLPattern matches routes, the Navigation API intercepts same-origin navigations (link clicks, back/forward) with built-in focus and scroll handling.

• keep the router mechanics in router.js; app.js only declares the route table
• define routes mapping paths to custom element tag names
• named path parameters (/items/:id) are passed to the routed component as attributes
• import all routed components before initializing the router
• use standard <a href="..."> for navigation — no programmatic routing unless necessary
• non-matching and cross-origin URLs must fall through to regular browser navigation
• never intercept reloads (event.navigationType === 'reload') — intercepting would turn them into same-document re-renders with stale modules and in-memory state; reload means reload
• serve the app with an index.html fallback for unknown paths (deep links)

// router.js
export const initRouter = (outlet, routeConfig) => {
    const routes = routeConfig.map(({ path, component }) => ({
        pattern: new URLPattern({ pathname: path }),
        component
    }));

    const render = url => {
        const match = routes
            .map(({ pattern, component }) => ({ result: pattern.exec(url), component }))
            .find(({ result }) => result);
        if (!match) return false;
        const routeComponent = document.createElement(match.component);
        Object.entries(match.result.pathname.groups)
            .filter(([, value]) => value !== undefined)
            .forEach(([name, value]) => routeComponent.setAttribute(name, value));
        outlet.replaceChildren(routeComponent);
        return true;
    };

    navigation.addEventListener('navigate', event => {
        if (!event.canIntercept || event.hashChange || event.downloadRequest) return;
        if (event.navigationType === 'reload') return;
        const url = new URL(event.destination.url);
        if (!routes.some(({ pattern }) => pattern.test(url))) return;
        event.intercept({ handler: () => render(url) });
    });

    render(new URL(location.href));
}

// app.js
import { initRouter } from "./router.js";

initRouter(document.querySelector('.view'), [
  { path: '/',          component: 'b-items' },
  { path: '/add',       component: 'b-items-add' },
  { path: '/edit/:id',  component: 'b-items-add' }
]);

URLPattern uses the same :param syntax as router libraries (both inherit it from path-to-regexp). A routed component reads its parameters with this.getAttribute('id') and loads the matching entity through the control layer.

View Transitions (Same-Document) — Optional

Route changes may animate with same-document view transitions — the SPA counterpart of web-static's cross-document @view-transition. The reference implementation renders instantly; add transitions only when the application calls for them, by wrapping render:

const reducedMotion = matchMedia('(prefers-reduced-motion: reduce)');
const transitionTo = url => {
    if (!document.startViewTransition || reducedMotion.matches) return render(url);
    return document.startViewTransition(() => render(url)).finished;
};
// in the navigate listener: event.intercept({ handler: () => transitionTo(url) });

• same-document view transitions are Baseline Newly Available — the document.startViewTransition existence check is the required feature detection; browsers without it render instantly
• skip the transition under prefers-reduced-motion: reduce — reduced motion means instant route changes
• give elements that persist across routes (header, logo) a view-transition-name for continuity; each name must be unique within the rendered page
• customize the animation in CSS via ::view-transition-old(root) / ::view-transition-new(root) — the default cross-fade needs no CSS at all

HTML Structure

• single index.html entry point with semantic HTML (<main>, <header>, <nav>, <section>, <footer>)
• declare import maps in <head> to map bare module specifiers to local vendored files
• use root-absolute URLs (/style.css, /app.js, /libs/...) for all resources — with the index.html fallback the document is also served for nested routes like /edit/17, where relative URLs would resolve against /edit/ and 404
• load init.js (non-module) before app.js (module) only when a dependency needs globals — Redux Toolkit requires window.process.env.NODE_ENV; with reduction.js no globals are needed
• the router outlet is a <section> element — router replaces its content with route components
• use native <dialog> with showModal() for modal dialogs (Widely Available) — focus trapping, Esc dismissal, and ::backdrop styling come for free; close with dialog.close() or <form method="dialog">

<script type="importmap">
{
  "imports": {
    "lit-html": "/libs/lit-html.js",
    "@reduxjs/toolkit": "/reduction.js"
  }
}
</script>

CSS Rules

Design tokens, logical properties, modern CSS features, accessibility, and the Baseline policy follow /web-conventions. This stack adds:

• no CSS framework — plain CSS on design tokens; Bulma was removed from the reference in favor of custom properties
• keep design-token custom properties (--color-primary, --spacing-md, …) in a dedicated tokens.css, loaded before style.css; tokens.css projects the root-level tokens.json (W3C Design Tokens)
• use CSS Grid for page-level layout with named grid areas
• use container queries (@container) over media queries for responsive design
• set container-type: inline-size on body
• use dvh units for viewport height (min-height: 100dvh)
• style custom elements directly by tag name (b-items { ... })
• keep custom styles in a separate style.css
• use flexbox for component-level layout

Dependency Management

• there is no build system — no Vite, no Rollup, no npm install in the application
• runtime dependencies are vendored as self-contained ES modules in app/src/libs/
• update by fetching the released module straight from the npm registry with a small script (e.g. update-lit-html.sh <version>: curl the tarball, extract the single lit-html.js file into libs/) — an update is a plain file copy
• only dependencies that ship as a single dependency-free ESM file qualify — if a library needs bundling to become self-contained, that is a signal against adopting it
• import maps in index.html map bare specifiers to the vendored local files
• the application source never references node_modules — always import through the import map

Testing

Browser-driven system tests come from /web-system-tests — a Playwright suite in a separate tests/ directory, run against Chromium, Firefox, and WebKit, locating through the accessibility tree (getByRole, getByLabel) because that is this stack's public surface. It owns the layout, the configuration (baseURL plus webServer against the dev server on port 3000), coverage, and the green oracle; the black-box contract above it belongs to /system-tests.

Event Handling in Templates

• use lit-html event bindings: @click, @keyup, @input
• destructure event objects to extract relevant data: ({ target: { name, value } })
• use event.preventDefault() for form submissions
• validate forms using native HTML validation: form.reportValidity() and form.checkValidity() — both only pay off when inputs declare standard types and constraint attributes (required, pattern, min/max) per web-conventions
• for rules markup cannot express, use setCustomValidity() on the input so the error surfaces through the same native reporting — do not build a parallel error display
• read submitted values with new FormData(form) instead of querying individual inputs

Form-Associated Custom Elements

Components rendering native inputs into their light DOM need nothing extra — the inputs participate in the surrounding <form> natively. Make a component form-associated (Baseline Widely Available) only when the element itself is the control — a composite input with no single native equivalent (rating widget, tag picker) — and must contribute to FormData and native validation:

• declare static formAssociated = true and call this.attachInternals() in the constructor
• report the current value with internals.setFormValue() on every change
• express validity with internals.setValidity() so errors surface through the same native reporting (reportValidity()) — the same rule as setCustomValidity() above: no parallel error display
• prefer composing standard inputs first — reach for form association only when no standard input fits

What NOT to Do

• do not use Shadow DOM unless encapsulation is explicitly required
• do not use TypeScript — plain ES modules with JSDoc type declarations only
• do not install dependencies in the application root
• do not import from node_modules paths — use import maps
• do not add CSS preprocessors (Sass, Less, PostCSS)
• do not add a CSS framework (Bulma, Tailwind, …) — plain CSS on design tokens
• do not add bundlers or a build system — dependencies are vendored, application code runs as-is
• do not use Vite as dev server — it resolves bare imports itself instead of honoring the import map; use a plain static server with index.html fallback
• do not use React, Angular, Vue, Svelte, or any component framework
• do not add Redux Toolkit as a default dependency — reduction.js covers the used API; vendored Redux Toolkit is an optional import-map switch
• do not add a routing library (Vaadin Router, page.js, …) — route with the Navigation API and URLPattern
• do not create package.json in the application root
• do not dispatch actions directly from boundary components — always go through control functions
• do not put business logic in boundary components — delegate to control layer
• do not hand-roll modal overlays from <div>s — use <dialog>