| name | cem-serve |
| description | Set up cem serve for custom element development. Use when the user asks to "start dev server", "set up cem serve", "preview elements", "run demos", or needs help configuring the dev server, import maps, or CSS transforms.
|
| tools | Read, Bash, Glob, Grep |
Set Up CEM Serve for Development
Configure and run cem serve to preview custom elements with live reload,
auto-generated import maps, and interactive knobs.
Best Practices
Package Dependencies
Runtime imports must be in dependencies, not devDependencies. The import
map generator only resolves packages from dependencies.
{
"dependencies": {
"lit": "^3.0.0"
},
"devDependencies": {
"@pwrs/cem": "^0.9.0"
}
}
Project Config
Create .config/cem.yaml with generate and serve sections:
generate:
files:
- elements/*/my-*.ts
output: custom-elements.json
designTokens:
spec: tokens/my-tokens.json
prefix: my-prefix
demoDiscovery:
fileGlob: elements/*/demo/*.html
urlPattern: /elements/:tag/demo/:demo.html
urlTemplate: /elements/{{.tag}}/demo/{{.demo}}/
serve:
transforms:
css:
include:
- "elements/**/*.css"
generate.files: globs for element source filesgenerate.output: manifest output pathgenerate.designTokens: DTCG token file and CSS custom property prefixgenerate.demoDiscovery: how to find and route demo filesserve.transforms.css.include: globs for CSS files to serve as JS modules (enables import styles from './el.css' with { type: 'css' })
Demo Files
Demos are HTML partials, not full documents. No <!DOCTYPE>, <html>, or
<head>. Include inline <script type="module"> and <style> as needed.
The index.html demo is the default — keep it minimal (simplest possible usage).
Additional demos show specific features or variants.
Use <meta itemprop="name"> and <meta itemprop="description"> on non-index
demos for display in the dev server UI.
For demo authoring conventions, see the write-demos skill.
Testing with Chromeless Mode
cem serve --rendering=chromeless strips the dev server UI, serving bare demo
HTML with import maps and transforms — ideal for browser-based testing with
Playwright or Puppeteer. Append ?rendering=chromeless to any demo URL for
per-request chromeless mode without restarting.
See the gen-tests skill for full test scaffolding with page objects against
chromeless demos.
Workflow
1. Generate the Manifest
cem generate
Verify demos appear without warnings. If you see "No URL configured for demo",
check that demoDiscovery.urlPattern uses URLPattern syntax (:param captures,
not regex).
2. Start the Server
cem serve
Default port is 8000. Use --port <n> to change.
3. Verify
Open http://localhost:8000. The UI should show element demos in the sidebar.
Check browser console for import errors — if a bare specifier fails, the
package needs to be in dependencies.
Troubleshooting
Bare specifier errors
Move the package from devDependencies to dependencies and reinstall.
Duplicate demo route warnings
Delete the cached manifest and regenerate: rm custom-elements.json && cem generate
Port in use
lsof -ti:8000 | xargs kill or use --port.
Stale manifest on background regen
cem serve regenerates the manifest in the background on file changes. If the
background regen produces a smaller/empty manifest, ensure generate.files in
the config matches your source globs — the background regen uses the config,
not CLI args.
