Enhance Code Emphasis

The enhanceCodeEmphasis source enhancer adds visual emphasis to specific lines in code examples by marking them with a data-hl attribute. This allows you to highlight important code patterns or call attention to specific sections in demos using simple comment annotations.

• Single-line emphasis with @highlight comments
• Multi-line emphasis blocks with @highlight-start / @highlight-end pairs
• Text highlighting within a line with @highlight-text "text"
• Optional descriptions in quoted strings (e.g., @highlight "description")
• Strong emphasis with ! at the end of descriptions (e.g., @highlight "We must do this!")
• Automatic strong emphasis for nested blocks - lines inside multiple emphasis ranges are automatically marked as strong
• Works with all code languages that support comments
• Compatible with both inline and block comment styles

import { enhanceCodeEmphasis } from '@mui/internal-docs-infra/pipeline/enhanceCodeEmphasis';

// Use as a source enhancer
const sourceEnhancers = [enhanceCodeEmphasis];

// Pass to CodeHighlighter or use with enhanceCode

The enhancer processes @highlight comments in your source files and adds data-hl to the corresponding line elements in the HAST tree.

Emphasize the line containing the comment:

export default function Button() {
  return (
    <button className="primary">Click me</button> {/* @highlight */}
  );
}

The line with the <button> element will be emphasized.

Add context about what's being emphasized:

export default function Component() {
  const [count, setCount] = useState(0); // @highlight "We track state"

  return <div>{count}</div>;
}

End a description with ! to mark it as strong emphasis (data-hl="strong"):

export default function Component() {
  const apiKey = process.env.API_KEY; // @highlight "We must provide the API key!"

  return <div>...</div>;
}

Strong emphasis is useful for highlighting critical code that users must pay attention to or modify.

Emphasize a range of lines between start and end markers:

export default function Component() {
  return (
    // @highlight-start
    <div>
      <h1>Heading 1</h1>
      <p>Some content</p>
    </div>
    // @highlight-end
  );
}

Lines 4-7 (from <div> through </div>) will be emphasized. The range starts at the first line after @highlight-start and ends at the last line before @highlight-end.

Explain what the emphasized section demonstrates:

export default function Component() {
  return (
    // @highlight-start "We add a heading with an h1"
    <div>
      <h1>Heading 1</h1>
    </div>
    // @highlight-end
  );
}

Highlight specific text within a line:

export default function Component() {
  return (
    <div>
      <h1>Heading 1</h1> {/* @highlight-text "Heading 1" */}
    </div>
  );
}

This wraps the specified text in a <mark> element, allowing you to highlight a specific word or phrase within a line rather than the entire line.

Highlight multiple texts within a single line by providing multiple quoted strings:

export default function Component() {
  return (
    <div>
      <h1 className="primary">Heading 1</h1> {/* @highlight-text "primary" "Heading 1" */}
    </div>
  );
}

Each quoted text is independently wrapped in a <mark> element.

Configure in your webpack loader or server-side loading:

import { enhanceCodeEmphasis } from '@mui/internal-docs-infra/pipeline/enhanceCodeEmphasis';
import { createLoadServerSource } from '@mui/internal-docs-infra/pipeline/loadServerSource';
import { loadCodeVariant } from '@mui/internal-docs-infra/pipeline/loadCodeVariant';

const sourceEnhancers = [enhanceCodeEmphasis];

For configurable padding, use the factory:

import { createEnhanceCodeEmphasis } from '@mui/internal-docs-infra/pipeline/enhanceCodeEmphasis';

const sourceEnhancers = [
  createEnhanceCodeEmphasis({
    paddingFrameMaxSize: 3,
    focusFramesMaxSize: 10,
  }),
];

Then pass to your loading pipeline:

// Create a loadSource that extracts emphasis and focus comments
const loadSource = createLoadServerSource({
  notableCommentsPrefix: ['@highlight', '@focus'],
  removeCommentsWithPrefix: ['@highlight', '@focus'],
});

// Use with loadCodeVariant
const { code } = await loadCodeVariant(url, variantName, variant, {
  loadSource,
  sourceEnhancers,
  sourceParser,
});

// Or with CodeHighlighter
<CodeHighlighter
  sourceEnhancers={sourceEnhancers}
  // ... other props
/>

// Or with useCode
const { selectedFile } = useCode(props, {
  sourceEnhancers,
});

The default comment prefixes are @highlight and @focus, exported as EMPHASIS_COMMENT_PREFIX and FOCUS_COMMENT_PREFIX:

import {
  EMPHASIS_COMMENT_PREFIX,
  FOCUS_COMMENT_PREFIX,
} from '@mui/internal-docs-infra/pipeline/enhanceCodeEmphasis';

console.log(EMPHASIS_COMMENT_PREFIX); // '@highlight'
console.log(FOCUS_COMMENT_PREFIX); // '@focus'

The enhancer recognizes these comment patterns:

Descriptions are provided as quoted strings:

// @highlight "We initialize the state"
const [value, setValue] = useState(0);

// @highlight-start "We render the component"
<div>
  <p>Content</p>
</div>;
// @highlight-end

By default, @highlight comments are stripped from the rendered code. To show the comment syntax in documentation (while still applying the emphasis), use the displayComments code block attribute:

```jsx displayComments
export default function Button() {
  return (
    <button className="primary">Click me</button> {/* @highlight */}
  );
}
```

This is useful for documentation pages where you want to show users the comment syntax itself.

You can emphasize multiple individual lines:

export default function Form() {
  const [name, setName] = useState(''); // @highlight
  const [email, setEmail] = useState(''); // @highlight

  return (
    <form>
      <input value={name} onChange={(e) => setName(e.target.value)} />
      <input value={email} onChange={(e) => setEmail(e.target.value)} /> {/* @highlight */}
    </form>
  );
}

The enhancer handles nested emphasis blocks using a stack-based algorithm. Lines that fall within multiple emphasis ranges are automatically marked with strong emphasis (data-hl="strong"):

export default function Component() {
  return (
    // @highlight-start "outer block"
    <div>
      <header>Title</header>
      {/* @highlight-start "inner block"  */}
      <main>
        <p>Content</p>
      </main>
      {/* @highlight-end  */}
      <footer>Footer</footer>
    </div>
    // @highlight-end
  );
}

In this example:

• The outer block lines (<div>, <header>, <footer>, </div>) get normal emphasis
• The inner block lines (<main>, <p>, </main>) get strong emphasis because they're nested within both ranges
• Position markers (data-hl-position) are preserved from the innermost range

Combine both patterns in the same file:

export default function Dashboard() {
  const [data, setData] = useState([]); // @highlight

  return (
    <div>
      <Header />
      {/* @highlight-start */}
      <Chart data={data} />
      <Table data={data} />
      {/* @highlight-end */}
      <Footer /> {/* @highlight */}
    </div>
  );
}

When any emphasis is present, the enhancer splits the code block's frame structure around highlighted lines. Each original frame is broken into typed sub-frames that CSS can target for collapsing, animating, or scrolling to the focused area.

Given a 7-line code block where line 4 is highlighted, the frames are restructured:

When padding is configured (see Configurable Padding), additional typed frames surround the focused highlight region:

Use createEnhanceCodeEmphasis to configure padding frames around the focused highlight region:

import { createEnhanceCodeEmphasis } from '@mui/internal-docs-infra/pipeline/enhanceCodeEmphasis';

const sourceEnhancers = [
  createEnhanceCodeEmphasis({
    paddingFrameMaxSize: 3,
    focusFramesMaxSize: 10,
  }),
];

When paddingFrameMaxSize is 0 (the default), no padding frames are created and only region frames (highlighted, focus, etc.) and normal frames are produced.

Use @padding N directly on emphasis directives to override padding for that region:

const important = loadData(); // @highlight @padding 1

// @highlight-start @focus @padding 4 "Primary preview area"
<Panel>
  <Widget />
</Panel>;
// @highlight-end

In this example, the focused block uses padding 4 while the single highlighted line uses padding 1.

Padding precedence is:

1. Per-directive @padding N on the focused region
2. createEnhanceCodeEmphasis({ paddingFrameMaxSize })

Use @min N on emphasis directives to override focusFramesMaxSize for the focused region:

// @highlight-start @focus @min 6
<form>
  <Field name="firstName" />
  <Field name="lastName" />
  <Field name="email" />
  <Field name="company" />
  <Field name="role" />
  <Field name="location" />
  <Field name="timezone" />
</form>
// @highlight-end

In this example, the focused region is constrained to a visible window of 6 lines when collapsed, regardless of the global focusFramesMaxSize option.

Focus max size precedence is:

1. Per-directive @min N on the focused region
2. createEnhanceCodeEmphasis({ focusFramesMaxSize })

When multiple highlight regions exist, padding is added around the first region by default. Add @focus to override which region receives padding:

const theme = getTheme(); // @highlight
const config = getConfig();

// @highlight-start @focus
<ThemeProvider theme={theme}>
  <App config={config} /> {/* @highlight-text "config" */}
</ThemeProvider>;
// @highlight-end

In this example, @focus directs the padding frames to surround the ThemeProvider block instead of the theme line.

@focus can be combined with any highlight directive:

• // @highlight @focus
• // @highlight-start @focus
• // @highlight-start @focus "We render the provider"

Use @focus, @focus-start, and @focus-end to mark a region as focused without adding line-level highlighting (data-hl). Lines in a standalone @focus region will be part of a focused frame but won't have visual emphasis on individual lines:

const theme = getTheme(); // @highlight
const config = getConfig();

// @focus-start
<ThemeProvider theme={theme}>
  <App config={config} />
</ThemeProvider>;
// @focus-end

In this example, the ThemeProvider block is focused (padding frames surround it), but only the theme line has line-level highlighting.

This is particularly useful when an ESLint rule like lintJavascriptDemoFocus auto-inserts focus markers — the auto-generated @focus-start/@focus-end marks the preview area without adding highlight styling, leaving @highlight for manual use.

Target frame types with CSS to style highlighted frames and create collapsible code blocks:

/* Highlighted frames get rounded corners and background */
.codeBlock .frame[data-frame-type='highlighted'],
.codeBlock .frame[data-frame-type='highlighted-unfocused'] {
  background: #ebe4ff;
  border-radius: 8px;
  margin: 0 6px;
  padding: 0 6px;
}

/* Normal frames and unfocused highlighted/focus frames are hidden by default (collapsed) */
.codeBlock .frame:not([data-frame-type]),
.codeBlock .frame[data-frame-type='highlighted-unfocused'],
.codeBlock .frame[data-frame-type='focus-unfocused'] {
  max-height: 0;
  overflow: hidden;
  opacity: 0;
  transition:
    max-height 0.3s cubic-bezier(0.5, 0, 0, 1),
    opacity 0.2s ease 0.1s;
}

/* When supported, use interpolate-size for smoother height animation */
@supports (interpolate-size: allow-keywords) {
  .codeBlock .frame:not([data-frame-type]),
  .codeBlock .frame[data-frame-type='highlighted-unfocused'],
  .codeBlock .frame[data-frame-type='focus-unfocused'] {
    interpolate-size: allow-keywords;
    max-height: unset;
    height: 0;
    overflow: clip;
    transition:
      height 0.3s ease,
      opacity 0.3s ease;
  }
}

/* Padding frames appear dimmed when collapsed */
.codeBlock .frame[data-frame-type='padding-top'],
.codeBlock .frame[data-frame-type='padding-bottom'] {
  opacity: 0.5;
  transition: opacity 0.3s ease;
}

/* When expanded, show all frames */
/* max-height = line height (18.5px) × max frame size (120 lines) = 2220px */
.expanded .codeBlock .frame:not([data-frame-type]),
.expanded .codeBlock .frame[data-frame-type='highlighted-unfocused'],
.expanded .codeBlock .frame[data-frame-type='focus-unfocused'] {
  max-height: 2220px;
  opacity: 1;
  transition:
    max-height 1.5s cubic-bezier(0.25, 0.46, 0.45, 0.94),
    opacity 0.15s ease;
}

@supports (interpolate-size: allow-keywords) {
  .expanded .codeBlock .frame:not([data-frame-type]),
  .expanded .codeBlock .frame[data-frame-type='highlighted-unfocused'],
  .expanded .codeBlock .frame[data-frame-type='focus-unfocused'] {
    max-height: unset;
    height: auto;
    overflow: clip;
    transition:
      height 0.3s ease,
      opacity 0.3s ease;
  }
}

.expanded .codeBlock .frame[data-frame-type='padding-top'],
.expanded .codeBlock .frame[data-frame-type='padding-bottom'] {
  opacity: 1;
}

/* Use indent level to shift highlighted/focus frames left when collapsed */
.codeBlock .frame[data-frame-type='highlighted'],
.codeBlock .frame[data-frame-type='focus'] {
  transition: transform 0.3s ease;
}

.codeBlock .frame[data-frame-type='highlighted'][data-frame-indent='1'],
.codeBlock .frame[data-frame-type='focus'][data-frame-indent='1'] {
  transform: translateX(-2ch);
}

.codeBlock .frame[data-frame-type='highlighted'][data-frame-indent='2'],
.codeBlock .frame[data-frame-type='focus'][data-frame-indent='2'] {
  transform: translateX(-4ch);
}

/* Reset indent shift when expanded */
.expanded .codeBlock .frame[data-frame-type='highlighted'],
.expanded .codeBlock .frame[data-frame-type='focus'] {
  transform: translateX(0);
}

When a code block has both visible frames (highlighted, focus, padding) and hidden frames (normal, unfocused), the <code> element receives a data-collapsible attribute. Use this to conditionally show an expand/collapse toggle — code blocks without collapsible frames won't render the button:

/* Hide toggle by default */
.toggle {
  display: none;
}

/* Show only when the code block has collapsible frames */
.pre code:has([data-collapsible]) ~ .toggle {
  display: block;
}

The following demo shows a code block that starts collapsed, revealing only the highlighted and padding frames. Click Expand to show the full source.

import * as React from 'react';
import { fetchUser } from './api';

interface User {
  name: string;
  email: string;
}

export function UserProfile({ id }: { id: string }) {
  const [user, setUser] = React.useState<User | null>(null);

  React.useEffect(() => {
    let cancelled = false;
    fetchUser(id).then((data) => {
      if (!cancelled) {
        setUser(data);
      }
    });
    return () => {
      cancelled = true;
    };
  }, [id]);

  if (!user) {
    return <p>Loading...</p>;
  }

  return (
    <div>
      <h2>{user.name}</h2>
      <p>{user.email}</p>
    </div>
  );
}

import * as React from 'react';
import type { Code as CodeType } from '@mui/internal-docs-infra/CodeHighlighter/types';
import { parseImportsAndComments } from '@mui/internal-docs-infra/pipeline/loaderUtils';
import { EMPHASIS_COMMENT_PREFIX } from '@mui/internal-docs-infra/pipeline/enhanceCodeEmphasis';
import { Code } from './Code';

const source = `import * as React from 'react';
import { fetchUser } from './api';

interface User {
  name: string;
  email: string;
}

export function UserProfile({ id }: { id: string }) {
  const [user, setUser] = React.useState<User | null>(null);

  // @highlight-start
  React.useEffect(() => {
    let cancelled = false;
    fetchUser(id).then((data) => {
      if (!cancelled) {
        setUser(data);
      }
    });
    return () => {
      cancelled = true;
    };
  }, [id]);
  // @highlight-end

  if (!user) {
    return <p>Loading...</p>;
  }

  return (
    <div>
      <h2>{user.name}</h2>
      <p>{user.email}</p>
    </div>
  );
}`;

export async function CollapsibleCode() {
  const { code: strippedSource, comments } = await parseImportsAndComments(source, '/demo.tsx', {
    removeCommentsWithPrefix: [EMPHASIS_COMMENT_PREFIX],
    notableCommentsPrefix: [EMPHASIS_COMMENT_PREFIX],
  });

  const code: CodeType = {
    Default: {
      language: 'tsx',
      source: strippedSource!,
      comments,
    },
  };

  return <Code code={code} />;
}

A full demo component with a live preview, file tabs, and collapsible code. The code section starts collapsed, showing only the focused region.

'use client';

import * as React from 'react';

export function Counter() {
  const [count, setCount] = React.useState(0);

  return (
    <button type="button" onClick={() => setCount((c) => c + 1)}>
      Count: {count}
    </button>
  );
}

This demo uses @highlight on the formatDate call and standalone @focus-start/@focus-end around the useEffect block. The useEffect region is focused (padding frames surround it) but has no line-level highlighting, while the formatDate line has line-level highlighting but is unfocused.

import * as React from 'react';
import { formatDate } from './formatDate';
import { fetchEvents } from './fetchEvents';

const today = formatDate(new Date());

export function Calendar() {
  const [events, setEvents] = React.useState([]);

  React.useEffect(() => {
    fetchEvents(today).then(setEvents);
  }, []);

  return (
    <div className="calendar">
      <h2>{today}</h2>
      <ul>
        {events.map((event) => (
          <li key={event.id}>{event.title}</li>
        ))}
      </ul>
    </div>
  );
}

import * as React from 'react';
import type { Code as CodeType } from '@mui/internal-docs-infra/CodeHighlighter/types';
import { parseImportsAndComments } from '@mui/internal-docs-infra/pipeline/loaderUtils';
import {
  EMPHASIS_COMMENT_PREFIX,
  FOCUS_COMMENT_PREFIX,
} from '@mui/internal-docs-infra/pipeline/enhanceCodeEmphasis';
import { Code } from './Code';

const source = `import * as React from 'react';
import { formatDate } from './formatDate';
import { fetchEvents } from './fetchEvents';

const today = formatDate(new Date()); // @highlight

export function Calendar() {
  const [events, setEvents] = React.useState([]);

  // @focus-start
  React.useEffect(() => {
    fetchEvents(today).then(setEvents);
  }, []);
  // @focus-end

  return (
    <div className="calendar">
      <h2>{today}</h2>
      <ul>
        {events.map((event) => (
          <li key={event.id}>{event.title}</li>
        ))}
      </ul>
    </div>
  );
}`;

export async function FocusCode() {
  const { code: strippedSource, comments } = await parseImportsAndComments(source, '/demo.tsx', {
    removeCommentsWithPrefix: [EMPHASIS_COMMENT_PREFIX, FOCUS_COMMENT_PREFIX],
    notableCommentsPrefix: [EMPHASIS_COMMENT_PREFIX, FOCUS_COMMENT_PREFIX],
  });

  const code: CodeType = {
    Default: {
      language: 'tsx',
      source: strippedSource!,
      comments,
    },
  };

  return <Code code={code} />;
}

With no padding configured, only highlighted/focus and normal frames are produced. The data-frame-indent attribute on region frames tells CSS how far the code is indented. This demo highlights an import statement (indent 0) and uses standalone @focus-start/@focus-end around the deeply nested <DatePicker> usage (indent 3). When collapsed, the focused frame shifts left by 6ch so it aligns with the left edge, then resets when expanded.

import * as React from 'react';
import { DatePicker } from './DatePicker';

export function ScheduleView() {
  const [date, setDate] = React.useState(null);

  return (
    <main>
      <header>
        <h1>Schedule</h1>
      </header>
      <section>
        <form>
          <label htmlFor="date">Pick a date</label>
          <DatePicker
            id="date"
            value={date}
            onChange={setDate}
            minDate={new Date()}
            format="MM/dd/yyyy"
          />
        </form>
      </section>
      <footer>
        <p>All times shown in UTC</p>
      </footer>
    </main>
  );
}

import * as React from 'react';
import type { Code as CodeType } from '@mui/internal-docs-infra/CodeHighlighter/types';
import { parseImportsAndComments } from '@mui/internal-docs-infra/pipeline/loaderUtils';
import {
  EMPHASIS_COMMENT_PREFIX,
  FOCUS_COMMENT_PREFIX,
} from '@mui/internal-docs-infra/pipeline/enhanceCodeEmphasis';
import { CodeIndent } from './CodeIndent';

const source = `import * as React from 'react';
import { DatePicker } from './DatePicker'; // @highlight

export function ScheduleView() {
  const [date, setDate] = React.useState(null);

  return (
    <main>
      <header>
        <h1>Schedule</h1>
      </header>
      <section>
        <form>
          <label htmlFor="date">Pick a date</label>
          {/* @focus-start */}
          <DatePicker
            id="date"
            value={date}
            onChange={setDate}
            minDate={new Date()}
            format="MM/dd/yyyy"
          />
          {/* @focus-end */}
        </form>
      </section>
      <footer>
        <p>All times shown in UTC</p>
      </footer>
    </main>
  );
}`;

export async function IndentCode() {
  const { code: strippedSource, comments } = await parseImportsAndComments(source, '/demo.tsx', {
    removeCommentsWithPrefix: [EMPHASIS_COMMENT_PREFIX, FOCUS_COMMENT_PREFIX],
    notableCommentsPrefix: [EMPHASIS_COMMENT_PREFIX, FOCUS_COMMENT_PREFIX],
  });

  const code: CodeType = {
    Default: {
      language: 'tsx',
      source: strippedSource!,
      comments,
    },
  };

  return <CodeIndent code={code} />;
}

When focusFramesMaxSize is set and a highlighted region exceeds that limit, a focused window is taken from the start of the region, and the remaining overflow lines are marked as unfocused. This demo uses focusFramesMaxSize: 6 with the <form> JSX highlighted (10 lines). When collapsed, only the first 6 lines of the region are visible; expanding reveals the full highlighted region with the overflow lines.

import * as React from 'react';

interface FormData {
  name: string;
  email: string;
  message: string;
}

export function ContactForm() {
  const [form, setForm] = React.useState<FormData>({
    name: '',
    email: '',
    message: '',
  });

  const handleSubmit = async (event: React.FormEvent) => {
    event.preventDefault();
    const response = await fetch('/api/contact', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify(form),
    });
    if (!response.ok) {
      throw new Error('Failed to submit');
    }
    setForm({ name: '', email: '', message: '' });
  };

  return (
    <form onSubmit={handleSubmit}>
      <label htmlFor="name">Name</label>
      <input id="name" value={form.name} onChange={(e) => setForm({ ...form, name: e.target.value })} />
      <label htmlFor="email">Email</label>
      <input id="email" value={form.email} onChange={(e) => setForm({ ...form, email: e.target.value })} />
      <label htmlFor="message">Message</label>
      <textarea id="message" value={form.message} onChange={(e) => setForm({ ...form, message: e.target.value })} />
      <button type="submit">Send</button>
    </form>
  );
}

import * as React from 'react';
import type { Code as CodeType } from '@mui/internal-docs-infra/CodeHighlighter/types';
import { parseImportsAndComments } from '@mui/internal-docs-infra/pipeline/loaderUtils';
import { EMPHASIS_COMMENT_PREFIX } from '@mui/internal-docs-infra/pipeline/enhanceCodeEmphasis';
import { CodeMaxSize } from './CodeMaxSize';

const source = `import * as React from 'react';

interface FormData {
  name: string;
  email: string;
  message: string;
}

export function ContactForm() {
  const [form, setForm] = React.useState<FormData>({
    name: '',
    email: '',
    message: '',
  });

  const handleSubmit = async (event: React.FormEvent) => {
    event.preventDefault();
    const response = await fetch('/api/contact', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify(form),
    });
    if (!response.ok) {
      throw new Error('Failed to submit');
    }
    setForm({ name: '', email: '', message: '' });
  };

  return (
    // @highlight-start
    <form onSubmit={handleSubmit}>
      <label htmlFor="name">Name</label>
      <input id="name" value={form.name} onChange={(e) => setForm({ ...form, name: e.target.value })} />
      <label htmlFor="email">Email</label>
      <input id="email" value={form.email} onChange={(e) => setForm({ ...form, email: e.target.value })} />
      <label htmlFor="message">Message</label>
      <textarea id="message" value={form.message} onChange={(e) => setForm({ ...form, message: e.target.value })} />
      <button type="submit">Send</button>
    </form>
    // @highlight-end
  );
}`;

export async function MaxSizeCode() {
  const { code: strippedSource, comments } = await parseImportsAndComments(source, '/demo.tsx', {
    removeCommentsWithPrefix: [EMPHASIS_COMMENT_PREFIX],
    notableCommentsPrefix: [EMPHASIS_COMMENT_PREFIX],
  });

  const code: CodeType = {
    Default: {
      language: 'tsx',
      source: strippedSource!,
      comments,
    },
  };

  return <CodeMaxSize code={code} />;
}

When @highlight is used without @focus, the enhancer wraps highlighted lines in a frame with data-frame-type="highlighted". Style these frames with background and rounded corners:

/* Highlighted frames get rounded corners and background */
.frame[data-frame-type='highlighted'],
.frame[data-frame-type='highlighted-unfocused'] {
  background: #ebe4ff;
  border-radius: 8px;
  margin: 0 6px;
  padding: 0 6px;
}

The margin: 0 6px pulls the frame inward from the padding: 0 12px on the parent .frame, so highlighted frames appear visually inset. The padding: 0 6px ensures child content still has horizontal spacing.

When @highlight lines appear inside a @focus region, or when highlights are nested, the individual lines receive data-hl attributes. Style these with negative margins that expand into the parent frame's padding:

/* Required: Display lines as blocks for line-based highlighting */
.frame[data-lined] {
  display: block;
  white-space: normal;
}

.frame[data-lined] .line {
  display: block;
  white-space: pre;
}

/* Line-level highlight inside a frame (nested emphasis) */
.line[data-hl] {
  background: #ebe4ff;
  margin: 0 -6px;
  padding: 0 6px;
}

/* Text-level highlight (from @highlight-text) */
mark {
  background: #ebe4ff;
  border-radius: 4px;
}

/* Text-level highlight inside a containing highlight range */
mark[data-hl] {
  background: #e1d9ff;
}

/* Text-level highlight inside 2+ nested highlight ranges */
mark[data-hl='strong'] {
  background: #d4c8ff;
}

/* Strong emphasis for nested highlights or descriptions ending with ! */
.line[data-hl='strong'] {
  background: #e1d9ff;
  margin: 0 -6px;
  padding: 0 6px;
}

/* Single-line emphasis - rounded on all corners */
.line[data-hl-position='single'] {
  border-radius: 8px;
}

/* Multiline block start - rounded top corners */
.line[data-hl-position='start'] {
  border-top-left-radius: 8px;
  border-top-right-radius: 8px;
}

/* Multiline block end - rounded bottom corners */
.line[data-hl-position='end'] {
  border-bottom-left-radius: 8px;
  border-bottom-right-radius: 8px;
}

Descriptions can appear on lines (for nested highlights) or frames (for standalone highlights).

Line-level descriptions (highlight inside focus, or nested/strong):

.line[data-hl-description]::after {
  content: attr(data-hl-description);
  float: right;
  background: #8145b5;
  border-radius: 8px;
  color: white;
  padding: 2px 4px;
  margin-right: -6px;
}

Frame-level descriptions (standalone highlights):

.frame[data-frame-description]::before {
  content: attr(data-frame-description);
  float: right;
  background: #8145b5;
  border-radius: 8px;
  color: white;
  padding: 2px 4px;
}

Use single-line emphasis for specific statements:

// ✓ Good - highlights the important line
const result = await fetchData();

// ✗ Avoid - too vague
function processData() {
  const result = await fetchData();
  return result;
}

Add context when the emphasis isn't obvious:

// ✓ Good - explains what to notice
<div className="container"> {/* @highlight "We add the container class" */}

// ✗ Less helpful - just marks the line
<div className="container"> {/* @highlight */}

Use multi-line emphasis for logical code blocks:

// ✓ Good - emphasizes the complete pattern
// @highlight-start "We handle the form submission"
<form onSubmit={handleSubmit}>
  <input type="text" value={name} onChange={handleChange} />
  <button type="submit">Submit</button>
</form>
// @highlight-end

Use the appropriate comment style for your language:

// JSX/TSX - use {/* */} for inline, // for block
<div>{/* @highlight */}</div>
// @highlight-start

// JavaScript/TypeScript - use //
const x = 42; // @highlight

// CSS - use /* */
.button {
  color: blue; /* @highlight */
}

Check that:

1. Comments are being extracted with notableCommentsPrefix: ['@highlight']
2. The comment syntax exactly matches the expected patterns
3. Multi-line pairs have matching start/end markers
4. Line numbers in the HAST tree match your source

If you have unmatched @highlight-start or @highlight-end:

• Extra @highlight-end → ignored (no effect)
• Extra @highlight-start → ignored (no matching end)

Check your code for balanced pairs.

Verify that:

• Comments are on the correct lines in your source
• Line numbers in HAST tree match (check dataLn properties)
• Comment extraction isn't removing the comments from the source

1. Comment Extraction: Comments are extracted during parsing with notableCommentsPrefix: ['@highlight', '@focus']
2. Directive Parsing: The enhancer scans comments for @highlight, @highlight-start, @highlight-end, @highlight-text, @focus, @focus-start, @focus-end, and optional @padding N / @min N modifiers
3. Line Calculation: Single-line directives mark their own line; multi-line directives mark all lines from the first line after @highlight-start to the last line before @highlight-end
4. Nested Detection: Lines inside multiple ranges are automatically marked as strong emphasis
5. HAST Modification: The enhancer traverses the HAST tree and adds dataHl: '' (or dataHl: 'strong' for nested lines or descriptions ending with !) to line elements. Text highlights (@highlight-text) wrap matched text in <mark> elements, which inherit data-hl from their containing highlight range via containingRangeDepth

Lines in the HAST tree have this structure after enhancement:

{
  type: 'element',
  tagName: 'span',
  properties: {
    className: 'line',
    dataLn: 3,                              // Line number
    dataHl: '',                             // Added by enhancer (or 'strong' for nested/!)
    dataHlDescription: 'We track state',    // Optional description
    dataHlPosition: 'single',               // 'single' | 'start' | 'end' for line position
  },
  children: [/* code tokens */]
}

For text highlighting with @highlight-text "text", the specified text within the line is wrapped in a <mark> element:

{
  type: 'element',
  tagName: 'mark',
  properties: {},
  children: [{ type: 'text', value: 'text' }]
}

When the <mark> is inside a containing highlight range, it inherits data-hl based on the nesting depth: data-hl="" for one containing range, data-hl="strong" for two or more. When the highlighted text matches a single syntax-highlighting <span>, the span is replaced in-place (its tagName becomes mark and highlight properties are merged) to keep the output flat.

The enhancer adds these attributes to line elements when the highlight is nested (inside a focus frame or strong from nesting):

For standalone highlights (no focus context, not nested), the visual emphasis is handled entirely at the frame level. The frame receives data-frame-description if a description was provided (see Frame Data Attributes).

For single-line emphasis with @highlight:

• data-hl is set (or data-hl="strong" if description ends with !)
• data-hl-description is set if a description was provided
• data-hl-position="single" is set to distinguish from multiline range boundaries

For multi-line emphasis with @highlight-start / @highlight-end:

• data-hl is set on all lines in the range (or data-hl="strong" if description ends with ! or line is nested)
• data-hl-description is set on the first line (if provided)
• data-hl-position="start" is set on the first line (only for ranges with more than one line)
• data-hl-position="end" is set on the last line (only for ranges with more than one line)

Note: The above line-level attributes are only applied when the highlight is inside a focus region or is nested (strong). For standalone highlights, descriptions are placed on the frame element as data-frame-description instead.

For text highlighting with @highlight-text "text":

• The specified text within the line is wrapped in a <mark> element
• When inside a containing highlight range, marks inherit data-hl based on nesting depth ("" for 1 range, "strong" for 2+)
• The line element itself does not receive any additional attributes unless it is also inside a focus region or nested

Multi-line directives are paired using a stack:

• @highlight-start pushes onto the stack
• @highlight-end pops from the stack and emphasizes the range
• Inner (nested) ranges are processed before outer ranges
• Lines that appear in multiple ranges are automatically marked as strong emphasis
• Position markers from inner ranges are preserved when outer ranges are processed

HastRoot

SourceComments | undefined

string

HastRoot | Promise<HastRoot>

type EMPHASIS_COMMENT_PREFIX = '@highlight'

type EmphasisMeta = {
  /** Optional description for this emphasis */
  description?: string;
  /** Position: 'single' for single-line, 'start'/'end' for multiline range bounds, undefined for middle */
  position?: 'single' | 'start' | 'end';
  /** Whether this is a strong emphasis (description ended with !) */
  strong?: boolean;
  /** For text highlighting: the specific texts to highlight within the line */
  highlightTexts?: string[];
  /** Whether this line's region is the focused region (for padding) */
  focus?: boolean;
  /** Whether the line itself should receive data-hl. True for highlight directives and false for focus-only directives. */
  lineHighlight: boolean;
  /** How many containing highlight ranges wrap this line (used for mark data-hl propagation) */
  containingRangeDepth?: number;
  /** Optional per-directive padding override for this region */
  paddingFrameMaxSize?: number;
  /** Optional per-directive focus max size override for this region */
  focusFramesMaxSize?: number;
  /**
   * True when the overrides were propagated from a multiline range
   * rather than set by an explicit per-line directive.
   * Explicit overrides take precedence over propagated ones in regions.
   */
  propagatedOverride?: boolean;
}

type EnhanceCodeEmphasisOptions = {
  /**
   * Maximum number of padding lines above and below the focused highlight region.
   * Padding frames provide surrounding context for the highlighted code.
   * Set to 0 or omit to disable padding frames.
   */
  paddingFrameMaxSize?: number;
  /**
 * Maximum total number of lines in the focus area (padding-top + focused region + padding-bottom).
 * Defaults to `12` when not explicitly configured.
 * @default 12
When the region fits within this limit, padding sizes are reduced so the total focus area
fits. The remainder after subtracting the region size is split: floor(remainder/2) for
padding-top and ceil(remainder/2) for padding-bottom.
When the region exceeds this limit, a focused window is taken from the start of the
region, and the remaining overflow lines are marked as unfocused.
 */
  focusFramesMaxSize?: number;
  /**
   * When `true`, throws an error if a `@highlight-text` match has to be
   * fragmented across element boundaries (producing `data-hl-part` spans).
   * Wrapping multiple complete elements in a single `data-hl` span is still
   * allowed — only boundary-straddling matches are rejected.
   */
  strictHighlightText?: boolean;
}

type FOCUS_COMMENT_PREFIX = '@focus'

type FrameRange = {
  /** First line number (1-based, inclusive) */
  startLine: number;
  /** Last line number (1-based, inclusive) */
  endLine: number;
  /** The type of frame */
  type:
    | 'normal'
    | 'padding-top'
    | 'highlighted'
    | 'highlighted-unfocused'
    | 'focus'
    | 'focus-unfocused'
    | 'padding-bottom'
    | 'comment';
  /** Index of the highlighted region this frame belongs to. Present on region-type frames. */
  regionIndex?: number;
  /**
   * Present on frames created by splitting an oversized region via `focusFramesMaxSize`.
   * - `'visible'` — the focused window kept visible when collapsed.
   * - `'hidden'`  — the overflow portion hidden when collapsed.
   */
  truncated?: 'visible' | 'hidden';
}

type MIN_COMMENT_PREFIX = '@min'

type PADDING_COMMENT_PREFIX = '@padding'

• Parse Source - Syntax highlighting and HAST generation
• HAST Utilities - Converting HAST to React JSX
• Load Server Source - Loading source files with enhancers
• Code Highlighter - Displaying code with emphasis