Style the custom-element host directly from page CSS, and use the component’s public styling hooks—CSS custom properties, named parts, and slots—to customize its contents. Shadow DOM blocks ordinary page selectors from reaching arbitrary internal elements, so keep that boundary intact and expose only the styling options consumers need.
What CSS can reach across a shadow boundary?
When a custom element uses Shadow DOM, its shadow tree has a separate CSS scope. A page rule can match the host element, such as my-element, but it cannot select an arbitrary button inside that element with a selector such as my-element button. Styles defined inside the shadow tree do not ordinarily leak out to the page either. MDN summarizes the boundary this way: “The page CSS does not affect nodes inside the shadow DOM.” See MDN’s guide to using Shadow DOM and its CSS scoping guide.
This separation is useful: component authors can change internal markup without unexpectedly breaking consumer styles. Customization still works, but it should use deliberate public hooks rather than selectors that depend on private structure.
Choose the right customization hook
| What you need to customize | Use | What it exposes |
|---|---|---|
| Theme values such as color, spacing, or font choices | CSS custom properties | A small set of values, without exposing internal elements. |
| A specific internal control or label | part and consumer-side ::part() |
Only the internal elements explicitly named as parts. |
| Content supplied by the page | <slot>, optionally with ::slotted() rules |
Consumer-provided light-DOM content. |
| Defaults or state styling on the host | :host and :host(...) |
The custom-element host, not its internal descendants. |
| Styling a nested component’s selected part from outside a wrapper | exportparts |
Only the part names the wrapper deliberately forwards. |
Prefer custom properties for stable theme values, add parts when consumers need precise control of selected elements, and use slots when consumers own the inserted content. Use exportparts only when a wrapper should intentionally pass a nested component’s styling hook through another shadow boundary.
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Style the host from the page or inside the component
Consumers can style the host as an ordinary element. The component can set its own host defaults with :host and target host attributes or other supported conditions with :host(...). These rules affect the host, not arbitrary nodes in its shadow tree. See MDN’s reference for :host().
Component-side host defaults
:host {
display: block;
color: var(--my-element-color, #222);
}
:host([emphasis]) {
font-weight: 700;
}The first rule establishes a default display and consumes a theme token with a fallback. The second styles the host when it has the emphasis attribute.
Consumer-side host styling
my-element {
margin-block: 1rem;
--my-element-color: rebeccapurple;
}Use host selectors for host-level layout and for setting documented custom properties. Avoid selectors that assume a particular internal tag or markup arrangement.
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Theme internals with CSS custom properties
CSS custom properties with names such as --my-element-color inherit by default, which makes them useful for passing theme values into a shadow tree. The component reads the property in its own stylesheet and can provide a fallback with var(--property, fallback). See MDN’s guide to CSS custom properties.
/* Inside the component's shadow stylesheet */
.action {
color: var(--my-element-color, #222);
}/* In the page stylesheet */
my-element {
--my-element-color: rebeccapurple;
}Document the supported tokens and their meaning. A short, stable token set gives consumers useful theme control while allowing the component to change its internal structure.
Expose selected internals with parts
When a consumer needs to style a particular internal element—not just supply a theme value—the component can mark that element with a part name. The page can then target it using ::part(name). This is an explicit styling API: consumers can reach the named part, not gain general selector access to its descendants. See MDN’s guide to CSS shadow parts.
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/* Inside the component's shadow markup */
<button part="action">Save</button>/* In the page stylesheet */
my-element::part(action) {
border-radius: 0.5rem;
}Expose only parts consumers are expected to style. A component that documents no parts or custom-property contract has not necessarily provided a way to style its internals; do not assume that an internal element is reachable just because its markup is familiar.
Forward a nested part only when needed
A part inside a nested component does not automatically become reachable through every enclosing shadow root. A wrapper must deliberately forward selected names using exportparts if outside consumers should style them. Consult MDN’s exportparts reference for the attribute syntax.
Use slots for consumer-owned content
Content assigned to a slot remains in the consumer’s light DOM. Ordinary page CSS can style those supplied elements, and they retain the consumer’s tree context, including inheritance from their parent. From inside the component, ::slotted() can target assigned nodes, but it is a limited hook rather than a way to select arbitrary descendants inside those nodes. See MDN’s guide to templates and slots and its CSS scoping guide.
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Use slots when the consumer should provide the markup or text itself. Use custom properties or parts when the component owns the internal element and is offering a specific way to customize it.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose how to deliver shadow styles
For component styles, a <style> in a template is a direct, declarative option. A constructed CSSStyleSheet assigned through adoptedStyleSheets can be shared among multiple DOM trees; MDN describes this as creating one stylesheet and sharing it among many roots. See MDN’s Shadow DOM guide.
A stylesheet added with <link> inside a shadow tree has a rendering caveat: MDN notes that it does not block the shadow root’s paint, so content may briefly appear before those styles load. Consider that possibility when choosing external stylesheet delivery. See MDN’s guide to using custom elements.
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Do not treat a closed shadow root as a styling solution
With mode: "closed", accessing host.shadowRoot returns null. That setting does not provide a stronger CSS boundary than Shadow DOM already does, and MDN cautions that it is not a strong security mechanism. Keep the distinction clear: the shadow boundary scopes CSS; closed mode affects access through that property. See MDN’s Shadow DOM documentation.
A practical design rule for component authors
- Set component defaults and host states with
:host. - Offer custom properties for theme values expected to remain stable.
- Expose named parts only for internal elements consumers have a real reason to style.
- Use slots for content supplied and owned by the consumer.
- Forward nested parts with
exportpartsonly when the wrapper intends to support that customization.
This keeps the implementation encapsulated while making the supported customization contract visible and intentional.
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