content-warning Web Component

A web component for block and inline content warnings.

Based on the original concept by Aaron Gustafson.

Demo

Additional demos:

ESM CDN Demo (Source)
Unpkg CDN Demo (Source)

Installation

npm install @aarongustafson/content-warning

Usage

Option 1: Auto-define the custom element (easiest)

Import the package to automatically define the <content-warning> custom element:

import '@aarongustafson/content-warning';

Or use the define-only script in HTML:

<script
  src="./node_modules/@aarongustafson/content-warning/define.js"
  type="module"
></script>

Option 2: Import the class and define manually

Import the class and define the custom element with your preferred tag name:

import { ContentWarningElement } from '@aarongustafson/content-warning/content-warning.js';

customElements.define('my-custom-name', ContentWarningElement);

Basic Example

<!-- Block content warning -->
<content-warning type="violence spoilers">
  <p>This content contains violence and spoilers for the series finale.</p>
</content-warning>

<!-- Inline content warning -->
<p>
  The character dies in
  <content-warning type="spoilers" inline>episode 5</content-warning>.
</p>

Reader Mode Safety

Important: Reader Mode parsers (Edge Immersive Reader, Safari Reader, etc.) extract content before JavaScript runs. To prevent Reader Mode from extracting content inside <content-warning> elements, add the hidden attribute:

<content-warning type="violence" hidden>
  <p>This content will be hidden from Reader Mode.</p>
</content-warning>

How it works:

Before JS runs: The hidden attribute hides the element → Reader Mode skips it entirely
When component initializes: The component automatically removes the hidden attribute
After initialization: The component's Shadow DOM hiding logic takes over

Without the hidden attribute, Reader Mode will extract and display the content even though it appears hidden in the normal page view.

Note: The hidden attribute is automatically removed when the component boots, so you don't need to manage it yourself.

Content Hiding Modes

The component offers two modes for hiding content, each with different trade-offs:

Default Mode (Recommended)

Reader Mode Safe - Content is truly hidden from Reader Mode and screen readers until revealed.

<content-warning type="sensitive content">
  <p>This content is completely hidden until revealed.</p>
</content-warning>

Uses hidden and inert attributes on the content wrapper
Content is not extracted by Reader Mode (Edge, Safari, etc.)
Content is hidden from screen readers via native attributes
Best for sensitive content that should be truly hidden

Blur Mode

Visual Only - Content is visually obscured but still present in the DOM.

<content-warning type="spoilers" blur>
  <p>This content is blurred but technically visible in the DOM.</p>
</content-warning>

Uses CSS filter: blur() to obscure content visually
Uses aria-hidden="true" to hide from screen readers
Content may be extracted by Reader Mode (not guaranteed to be hidden)
Customizable blur amount via --content-warning-blur-amount CSS property
Best for aesthetic preference when complete hiding isn't critical

Trade-off Summary:

Without blur: Maximum safety and hiding (recommended for sensitive content)
With blur: Visual obscuring effect (not fully hidden from all contexts)

Attributes

Attribute	Type	Default	Description
`type`	`string`	`"content"`	Space-separated list of warning types (e.g., "violence spoilers nsfw")
`label-prefix`	`string`	`"Content Warning"`	The prefix text for the warning label
`label-suffix`	`string`	`"Click to reveal"`	The suffix text for the warning label. Set to `"false"` to hide.
`inline`	`boolean`	`false`	Display the warning inline instead of as a block overlay
`blur`	`boolean`	`false`	Use blur visual effect instead of complete hiding (NOT Reader Mode safe)

Default Button Label Format: {prefix}: {type} {suffix}

Example: "Content Warning: violence spoilers Click to reveal"

Note: Punctuation and spacing between label parts are controlled via CSS pseudo-elements (:after and :before), making them easy to customize without affecting the underlying text content.

Events

The component fires custom events that you can listen to:

Event	Description	Detail
`content-warning:revealed`	Fired when the user reveals the content by clicking the button	`{ type: string }` - The type of content warning

Example Event Handling

const element = document.querySelector('content-warning');

element.addEventListener('content-warning:revealed', (event) => {
  console.log('Content revealed:', event.detail.type);
  // Track analytics, log user action, etc.
});

Properties

Property	Type	Description
`type`	`string`	Get/set the warning type(s)
`labelPrefix`	`string`	Get/set the prefix text for the warning label
`labelSuffix`	`string`	Get/set the suffix text for the warning label
`revealed`	`boolean` (read-only)	Whether the content has been revealed

CSS Custom Properties

Customize the component's appearance with CSS variables:

Property	Default	Description
`--content-warning-color`	`#fff`	Outline color for focus state
`--content-warning-blur-amount`	`10px`	Amount of blur in blur mode (e.g., `5px`, `20px`)

Example

content-warning {
  --content-warning-color: #ff6b6b;
  --content-warning-blur-amount: 15px;
}

Shadow Parts

You can style internal elements using CSS Shadow Parts:

Part	Description
`overlay`	The full-area overlay div that covers the content
`button`	The warning button element inside the overlay
`label-prefix`	The prefix text span (e.g., "Content Warning")

Example Styling

/* Style the full-area overlay */
content-warning::part(overlay) {
  background: rgba(139, 0, 0, 0.95);
}

/* Style the button inside the overlay */
content-warning::part(button) {
  color: #fff;
  border: 3px solid #ff6b6b;
  padding: 2rem;
  font-size: 1.25rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  background: rgba(0, 0, 0, 0.5);
  border-radius: 0.5rem;
}

/* Style individual label parts */
content-warning::part(label-prefix) {
  font-weight: bold;
}

content-warning::part(label-type) {
  font-style: italic;
  color: #ff6b6b;
}

content-warning::part(label-suffix) {
  font-size: 0.875em;
  opacity: 0.9;
}

/* Style inline warnings differently */
content-warning[inline]::part(overlay) {
  background: #333;
}

content-warning[inline]::part(button) {
  padding: 0.5rem 0.75rem;
  border-radius: 0.25rem;
}

Internationalization (i18n)

Customize the button label for different languages using label-prefix and label-suffix attributes:

<!-- Spanish -->
<content-warning
  type="violencia gore"
  label-prefix="Advertencia de Contenido"
  label-suffix="Haz clic para revelar"
>
  <img src="image.jpg" alt="Sensitive image" />
</content-warning>

<!-- French -->
<content-warning
  type="contenu sensible"
  label-prefix="Avertissement"
  label-suffix="Cliquez pour révéler"
>
  <p>Contenu en français...</p>
</content-warning>

<!-- No suffix -->
<content-warning type="graphic content" label-suffix="false">
  <p>Content without suffix text</p>
</content-warning>

Style each label part individually:

content-warning::part(label-prefix) {
  font-weight: bold;
}

content-warning::part(label-type) {
  font-style: italic;
  color: #ff6b6b;
}

content-warning::part(label-suffix) {
  font-size: 0.875em;
  opacity: 0.9;
}
```

### Customizing Punctuation

Punctuation and spacing between label parts are controlled via CSS pseudo-elements:

```css
/* Default punctuation (already applied) */
.label-prefix::after {
  content: ": "; /* Colon and space after prefix */
}

[part="label-type"]::before {
  content: " "; /* Space before type */
}

[part="label-suffix"]::before {
  content: " "; /* Space before suffix */
}

/* Customize punctuation */
content-warning::part(button) .label-prefix::after {
  content: " — "; /* Em dash instead of colon */
}

/* Remove punctuation entirely */
content-warning::part(button) .label-prefix::after {
  content: " "; /* Just a space */
}
```adding: 0.5rem 0.75rem;
  border-radius: 0.25rem;
}

Accessibility

The component follows accessibility best practices:

Uses a semantic <button> element for the warning interaction
Button is keyboard accessible by default
Content is hidden from screen readers until revealed (both modes use aria-hidden or hidden attribute)
Default mode: Uses hidden + inert attributes (Reader Mode safe)
Blur mode: Uses aria-hidden="true" (visual obscuring only)
Sets role="alert" on the host element when content is revealed
Clones revealed content into a persistent role="alert" region for screen reader announcement
Screen readers announce the revealed content automatically
Focuses the revealed content for additional context

Browser Support

This component uses modern web standards:

Custom Elements v1
Shadow DOM v1
ES Modules

For older browsers, you may need polyfills.

Development

# Install dependencies
npm install

# Run tests
npm test

# Run tests with coverage
npm run test:coverage

# Lint code
npm run lint

# Format code
npm run format

# View demo
open demo/index.html