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Use a Web Component when a piece of interface needs reusable behavior that native HTML does not already provide. Web Components are not a reason to wrap every fragment of markup in a custom tag. A sound default order is: start with semantic native HTML, add a custom element when the reusable behavior justifies it, add Shadow DOM only when encapsulation solves a concrete problem, and use templates and slots when the structure or the consumer-provided content needs to vary.
What Web Components are made of
Web Components are a set of browser capabilities, not a single feature. They combine three parts that can be used together or separately:
- Custom elements, which add new tag names with behavior of their own.
- Shadow DOM, an optional scoped DOM tree attached to an element.
- Templates and slots (the
<template>and<slot>elements), which hold reusable structure and accept content supplied by the page author.
According to MDN’s documentation, a typical implementation follows four steps:
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HTMLElementand contains the component’s behavior. - Register the class with the custom element registry by calling
customElements.define(), which is thedefine()method ofCustomElementRegistry. - Optionally attach a Shadow DOM tree to the element.
- Optionally use
<template>and<slot>for structure and projected content.
After registration, the element is used in markup much like a built-in element. Shadow DOM is optional at every stage, so a custom element without any shadow tree is a complete, valid choice.
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Start with native HTML
Before writing a custom element, check whether a native element already supplies the meaning and behavior you need. Native elements come with roles, names, states, and keyboard handling that you would otherwise have to build and maintain yourself. Common candidates include:
<button>for any action triggered by activation, including toggles that usearia-pressed.<details>and<summary>for a disclosure that shows and hides content.<dialog>for modal or non-modal dialog windows.<select>,<input type="date">, and other form controls for data entry.
Reach for a custom element when the native element cannot express the behavior you need, or when a set of native elements must be combined and reused in a way that would otherwise be repeated across pages. A custom element that only adds a class name or a layout wrapper around ordinary markup is usually better written as a CSS rule or a plain HTML fragment.
When should I use Web Components?
Web Components fit best when a component meets most of these conditions:
- It is reused in several places, possibly across frameworks or in content authored by other teams.
- It holds its own state or interaction logic, such as a tabbed panel, a combobox, or a range slider.
- It needs a declarative API that can be written in HTML and driven from JavaScript.
- It should keep working when placed inside different page layouts without its styling or DOM being disturbed.
If the component is a one-off presentational block with no behavior, the cost of an additional custom element, its registration, and its API surface is rarely justified.
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Should every custom element use Shadow DOM?
No. A custom element can work entirely in the light DOM, where its children are ordinary page content. Shadow DOM is worth adding when isolation prevents a real problem. It gives the component a scoped tree: page CSS does not select the component’s internal nodes, and styles defined inside the shadow tree do not leak onto the rest of the page. This reduces accidental coupling, but it also creates a styling boundary that the component’s consumers must cross deliberately.
The trade-offs of each approach are summarized below.
| Approach | Best used when | Main cost |
|---|---|---|
| Light DOM custom element (no shadow tree) | Consumers should style and inspect the markup freely, or the component is mostly a behavior layer over its children. | Page CSS can accidentally change the component’s internals, and internal class names can collide with the page. |
| Open Shadow DOM | Internal structure and styles must be isolated, and the component must expose deliberate styling hooks. | Consumers can only restyle what the author exposes, so the styling API must be designed and documented. |
Closed Shadow DOM (mode: "closed") |
Rarely needed. The component should stop ordinary page scripts from reaching shadowRoot through the usual property. |
It is not a security boundary (see below), and it makes debugging and some tooling harder. |
MDN explicitly states that mode: "closed" is not a strong security mechanism. It only signals that page scripts should not reach the component’s internals through the ordinary shadowRoot property. Do not use it to hide secrets or to enforce access control.
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When a component uses Shadow DOM, give consumers stable ways to adjust appearance. The W3C Technical Architecture Group points to two mechanisms:
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- CSS custom properties, which inherit through the shadow boundary, so a consumer can set a value such as
--card-accenton the host element and the component can read it. - CSS Shadow Parts, where the component marks an internal node with a
partattribute, and the consumer targets it with the::part()pseudo-element.
/* Inside the component's shadow tree */
<h2 part="title"><slot name="heading"></slot></h2>
/* In the page */
my-card::part(title) {
color: navy;
}Document each hook as part of the component’s public contract. An undocumented internal selector is an accidental API that the component’s author cannot safely change.
Design the API for the platform
The W3C Technical Architecture Group’s Guidelines for creating web platform compatible components (2018) recommends patterns that feel familiar to HTML authors. These are design guidance rather than formal conformance requirements. In practice, that means:
- Use consistent names and attributes, following the naming habits of built-in elements.
- Accept simple configuration declaratively, in attributes and child markup.
- Send data outward with events rather than by reaching into the page.
- Keep the HTML and JavaScript APIs aligned, so that a value set in markup and a value set through a property behave the same way.
Keep attributes and properties in sync
Boolean attributes are true by presence, not by value. <toggle-switch checked> is checked, and <toggle-switch checked="false"> is still checked because the attribute exists. The property should reflect the attribute, and setting the property should update the attribute:
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static observedAttributes = ["checked"];
get checked() {
return this.hasAttribute("checked");
}
set checked(value) {
if (value) {
this.setAttribute("checked", "");
} else {
this.removeAttribute("checked");
}
}
attributeChangedCallback() {
this.render();
}
render() {
// Update the visual state from this.checked
}
}
customElements.define("toggle-switch", ToggleSwitch);Send changes outward with events
When the user changes the component’s state, dispatch an event so that the page can respond without reading internal fields. Include the new value in detail and set bubbles when the event should reach ancestors:
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this.dispatchEvent(
new CustomEvent("change", {
bubbles: true,
detail: { checked: this.checked },
})
);Account for lifecycle timing
The W3C TAG cautions component authors not to assume that a custom element is already attached to the document when its constructor runs. An element can be created before it is inserted into the page, so the constructor should avoid depending on document context. Do work that needs the element to be in the document, such as reading layout or querying the surrounding page, in connectedCallback, and make cleanup safe in disconnectedCallback.
Preserve composition and fallback with slots
Slots let the page supply markup while the component provides the structure around it. A slotted card might look like this:
<user-card>
<img slot="avatar" src="ada.jpg" alt="Ada Lovelace">
<span slot="name">Ada Lovelace</span>
</user-card>Web.dev recommends slots for composability. It also notes that nested content remains visible and accessible in browsers that do not support custom elements. That is a useful progressive enhancement property for the content, but it does not mean the component’s behavior works without JavaScript or without custom element support. Treat the fallback as “the content still reads”, not “the widget still works”.
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How do I make a custom element accessible?
Once a component is built from a div or span rather than a native control, every accessibility responsibility that the browser would normally handle falls to you. W3C guidance for custom controls says that authors must supply these provisions when a native control is not suitable.
Expose names, roles, and states
Give the control an accessible name, a role, and the states that users need to perceive. A custom switch needs more than appearance:
<div role="switch" tabindex="0" aria-checked="false" aria-label="Notifications"></div>The aria-checked value must change when the switch is toggled. A visual change alone is not exposed to assistive technology.
Support keyboard operation
The W3C Technical Architecture Group states: “Interactive elements are focusable, and can be interacted with using a keyboard in addition to mouse/touch.” Design the keyboard model before writing the styling. For a custom switch, Space and Enter should toggle the state. Keyboard focus must also be able to leave the component through the keyboard. W3C says that if a component uses a nonstandard exit method, that method must be explained to users. A component that traps Tab focus inside itself is a failure unless the exit is documented and works.
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Notify assistive technology of changes
When a value or status changes without the user moving focus, assistive technology needs to be told. W3C guidance says that authors should notify assistive technology when values change. For state changes such as aria-expanded or aria-checked, update the attribute itself. For status messages, such as “3 results found”, use a live region so that the message is announced.
Test the result
W3C guidance on custom controls calls for testing accessibility support. Do not assume that an element that looks correct also works correctly. At a minimum:
- Complete the entire interaction with the keyboard alone, including focus entry and exit.
- Check the accessible name, role, and state of each control in the browser’s accessibility tree.
- Run the component with at least one screen reader and confirm that changes are announced.
Comparing the options at a glance
The table below summarizes the trade-offs described in MDN and W3C design guidance across the axes that matter most when choosing between approaches. It is a synthesis of those sources, not a published scoring framework.
| Axis | Native element | Custom element, light DOM | Custom element, Shadow DOM |
|---|---|---|---|
| Semantics and built-in behavior | Provided by the browser; authors still must use the element correctly | Authors must supply role, name, states, and keyboard behavior | Same as light DOM; the internal tree is hidden from page scripts through ordinary access |
| Encapsulation | Not applicable | Page CSS can affect internals | Internal DOM and CSS are scoped; consumers need hooks such as custom properties or parts |
| Composition | Fixed by the element | Children appear in the page as ordinary content | Consumers supply content through named or default slots |
| Styling for consumers | Standard CSS on the element | Standard CSS, including page CSS on internals | Through custom properties and ::part(), as documented by the component |
| Lifecycle and integration | Not applicable | Authors must not assume the element is connected in the constructor | Same timing rules apply; the shadow tree should be created before it is used |
Accessibility is the axis where custom elements most often fail. Choose a custom element only when you are prepared to own every row of the accessibility checklist above.
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