HTML Minifier Next

Your web page optimization precision tool: HTML Minifier Next (HMN) is a super-configurable, well-tested, JavaScript-based HTML minifier able to also handle in-document CSS, JavaScript, and SVG minification.

The project was based on HTML Minifier Terser (HMT), which in turn had been based on Juriy “kangax” Zaytsev’s HTML Minifier (HM); as of 2025, both HTML Minifier Terser and HTML Minifier had been unmaintained for several years. HMN offers additional features and has been optimized for speed. While an independent project, it is still backwards-compatible with HMT and HM.

Installation

From npm for use as a command-line app:

npm i -g html-minifier-next

Directly with npx (no installation required):

npx html-minifier-next --help

From npm for programmatic use:

General usage

CLI

Use html-minifier-next --help to check all available options:

Configuration file

You can use a configuration file to specify options. The file can be either JSON format or a JavaScript module that exports the configuration object:

JSON configuration example:

{
  "collapseWhitespace": true,
  "removeComments": true,
  "fileExt": "html,php",
  "ignoreDir": "libs,vendor"
}

JavaScript module configuration example:

module.exports = {
  collapseWhitespace: true,
  removeComments: true,
  fileExt: "html,php",
  ignoreDir: ["libs", "vendor"]
};

Node.js

ESM with Node.js ≥16.14:

import { minify } from 'html-minifier-next';

const result = await minify('<p title="example" id="moo">foo</p>', {
  removeAttributeQuotes: true,
  removeOptionalTags: true
});
console.log(result); // “<p title=example id=moo>foo”

CommonJS:

const { minify } = require('html-minifier-next');

(async () => {
  const result = await minify('<p title="example" id="moo">foo</p>', { preset: 'comprehensive' });
  console.log(result); // “<p id=moo title=example>foo”
})();

See the original blog post for details of how it works, descriptions of most options, testing results, and conclusions.

Presets

HTML Minifier Next provides presets for common use cases. Presets are pre-configured option sets that can be used as a starting point:

• conservative: Basic minification with whitespace collapsing, comment removal, and removal of select attributes.
• comprehensive: More advanced minification for better file size reduction, including relevant conservative options plus attribute quote removal, optional tag removal, and more.

To review the specific options set, presets.js lists them in an accessible manner.

Using presets:

# Via CLI flag
html-minifier-next --preset conservative input.html

# Via config file
html-minifier-next --config-file=html-minifier.json input.html
# where html-minifier.json contains: { "preset": "conservative" }

# Override preset options
html-minifier-next --preset conservative --remove-empty-attributes input.html

Priority order: Presets are applied first, then config file options, then CLI flags. This allows you to start with a preset and customize as needed.

Options quick reference

Most of the options are disabled by default. Experiment and find what works best for you and your project.

Options can be used in config files (camelCase) or via CLI flags (kebab-case with -- prefix). Options that default to true use --no- prefix in CLI to disable them.

Sorting attributes and style classes

Minifier options like sortAttributes and sortClassNames won’t impact the plain‑text size of the output. However, using these options for more consistent ordering improves the compression ratio for Gzip and Brotli used over HTTP.

CSS minification

When minifyCSS is set to true, HTML Minifier Next uses Lightning CSS to minify CSS in style elements and attributes. Lightning CSS provides excellent minification by default.

You can pass Lightning CSS configuration options by providing an object:

const result = await minify(html, {
  minifyCSS: {
    targets: {
      // Browser targets for vendor prefix handling
      chrome: 95,
      firefox: 90,
      safari: 14
    },
    unusedSymbols: ['unused-class', 'old-animation']
  }
});

Available Lightning CSS options when passed as an object:

• targets: Browser targets for vendor prefix optimization (e.g., { chrome: 95, firefox: 90 }).
• unusedSymbols: Array of class names, IDs, keyframe names, and CSS variables to remove.
• errorRecovery: Boolean to skip invalid rules instead of throwing errors. This is disabled by default in Lightning CSS, but enabled in HMN when the continueOnMinifyError option is set to true (the default). Explicitly setting errorRecovery in minifyCSS options will override this automatic behavior.
• sourceMap: Boolean to generate source maps.

For advanced usage, you can also pass a function:

const result = await minify(html, {
  minifyCSS: function(text, type) {
    // `text`: CSS string to minify
    // `type`: `inline` for style attributes, `media` for media queries, `undefined` for `<style>` elements
    return yourCustomMinifier(text);
  }
});

JavaScript minification

When minifyJS is set to true, HTML Minifier Next uses Terser by default to minify JavaScript in <script> elements and event attributes.

You can choose between different JS minifiers using the engine field:

const result = await minify(html, {
  minifyJS: {
    engine: 'swc', // Use SWC for faster minification
    // SWC-specific options here
  }
});

Available engines:

• terser (default): The standard JavaScript minifier with excellent compression
• swc: Rust-based minifier that’s significantly faster than Terser (requires separate installation)

To use SWC, install it as a dependency:

(Build-only users may want to install it as a dev dependency: npm i -D @swc/core.)

Important: Inline event handlers (e.g., onclick="return false") always use Terser regardless of the engine setting, as SWC doesn’t support bare return statements. This is handled automatically—you don’t need to do anything special.

You can pass engine-specific configuration options:

// Using Terser with custom options
const result = await minify(html, {
  minifyJS: {
    compress: {
      drop_console: true  // Remove console.log statements
    }
  }
});

// Using SWC for faster minification
const result = await minify(html, {
  minifyJS: {
    engine: 'swc'
  }
});

For advanced usage, you can also pass a function:

const result = await minify(html, {
  minifyJS: function(text, inline) {
    // `text`: JavaScript string to minify
    // `inline`: `true` for event handlers (e.g., `onclick`), `false` for `<script>` elements
    return yourCustomMinifier(text);
  }
});

SVG minification

When minifySVG is set to true, HTML Minifier Next uses SVGO to optimize inline SVG elements. Complete <svg> subtrees are extracted and processed as a block, enabling deep structural optimization:

const result = await minify(html, {
  minifySVG: true // Enable with SVGO defaults
});

You can pass custom SVGO options:

const result = await minify(html, {
  minifySVG: {
    plugins: [{
      name: 'preset-default',
      params: {
        overrides: {
          convertShapeToPath: false // Keep original shapes
        }
      }
    }]
  }
});

Important:

• SVG minification only applies within <svg> elements
• Case sensitivity and self-closing slashes are automatically preserved in SVG (regardless of global settings)
• For maximum compression, use minifySVG together with collapseWhitespace and other options

CSS, JavaScript, and SVG cache configuration

HTML Minifier Next uses in-memory caches to improve performance when processing multiple files or repeated content. The cache sizes can be configured for optimal performance based on your use case:

const result = await minify(html, {
  minifyCSS: true,
  minifyJS: true,
  minifySVG: true,
  // Configure cache sizes (in number of entries)
  cacheCSS: 750,  // CSS cache size, default: 500
  cacheJS: 250,   // JS cache size, default: 500
  cacheSVG: 100   // SVG cache size, default: 500
});

Via CLI flags:

html-minifier-next --minify-css --cache-css 750 --minify-js --cache-js 250 --minify-svg --cache-svg 100 input.html

Via environment variables:

export HMN_CACHE_CSS=750
export HMN_CACHE_JS=250
export HMN_CACHE_SVG=100
html-minifier-next --minify-css --minify-js --minify-svg input.html

Configuration file:

{
  "minifyCSS": true,
  "cacheCSS": 750,
  "minifyJS": true,
  "cacheJS": 250,
  "minifySVG": true,
  "cacheSVG": 100
}

When to adjust cache sizes:

• Single file processing: Default 500 is sufficient
• Batch processing: Increase to 1000 or higher for better cache hit rates
• Memory-constrained environments: Reduce to 200–300 to save memory
• Hundreds/thousands of files: Increase to 1000–2000 for optimal performance

Important:

• Cache locking: Caches are created on the first minify() call and persist for the process lifetime. Cache sizes are locked after first initialization—subsequent calls reuse the same caches even if different cacheCSS, cacheJS, or cacheSVG options are provided. The first call’s options determine the cache sizes.
• Zero values: Explicit 0 values are coerced to 1 (minimum functional cache size) to avoid immediate eviction. If you want to minimize memory usage, use a small number like 10 or 50 instead of 0.

The caches persist across multiple minify() calls, making them particularly effective when processing many files in a batch operation.

Minification comparison

Please see the Minifier Benchmarks project for details on how HMN compares to other minifiers.

Examples

CLI

Sample command line:

html-minifier-next --collapse-whitespace --remove-comments --minify-js --input-dir=. --output-dir=example

Example using npx:

npx html-minifier-next --input-dir=test --preset comprehensive --output-dir example

Process specific files and directories:

# Process default extensions (html, htm, xhtml, shtml)
html-minifier-next --collapse-whitespace --input-dir=src --output-dir=dist

# Process only specific extensions
html-minifier-next --collapse-whitespace --input-dir=src --output-dir=dist --file-ext=html,php

# Using configuration file that sets `fileExt` (e.g., `"fileExt": "html,php"`)
html-minifier-next --config-file=html-minifier.json --input-dir=src --output-dir=dist

# Process all files (explicit wildcard)
html-minifier-next --collapse-whitespace --input-dir=src --output-dir=dist --file-ext='*'

Exclude directories from processing:

# Ignore a single directory
html-minifier-next --collapse-whitespace --input-dir=src --output-dir=dist --ignore-dir=libs

# Ignore multiple directories
html-minifier-next --collapse-whitespace --input-dir=src --output-dir=dist --ignore-dir=libs,vendor,node_modules

# Ignore by relative path (only ignores src/static/libs, not other “libs” directories)
html-minifier-next --collapse-whitespace --input-dir=src --output-dir=dist --ignore-dir=static/libs

Dry run mode (preview outcome without writing files):

# Preview with output file
html-minifier-next input.html -o output.html --dry --collapse-whitespace

# Preview directory processing with statistics per file and total
html-minifier-next --input-dir=src --output-dir=dist --dry --collapse-whitespace
# Output: [DRY RUN] Would process directory: src → dist
#   index.html: 1,234 → 892 bytes (-342, 27.7%)
#   about.html: 2,100 → 1,654 bytes (-446, 21.2%)
# ---
# Total: 3,334 → 2,546 bytes (-788, 23.6%)

Verbose mode (show detailed processing information):

# Show processing details while minifying
html-minifier-next --input-dir=src --output-dir=dist --verbose --collapse-whitespace
# Output: CLI options: collapseWhitespace
#   ✓ src/index.html: 1,234 → 892 bytes (-342, 27.7%)
#   ✓ src/about.html: 2,100 → 1,654 bytes (-446, 21.2%)
# ---
# Total: 3,334 → 2,546 bytes (-788, 23.6%)

# `--dry` automatically enables verbose output
html-minifier-next --input-dir=src --output-dir=dist --dry --collapse-whitespace

Special cases

Ignoring chunks of markup

If you have chunks of markup you would like preserved, you can wrap them with <!-- htmlmin:ignore -->.

Minifying JSON content

JSON script types are minified automatically without configuration, including application/json, application/ld+json, application/manifest+json, application/vnd.geo+json, application/problem+json, application/merge-patch+json, application/json-patch+json, importmap, and speculationrules. Malformed JSON is preserved by default (with continueOnMinifyError: true).

Note: The processScripts option is only for script types containing HTML templates (e.g., text/ng-template, text/x-handlebars-template), not for JSON.

Preserving SVG and MathML elements

SVG and MathML elements are automatically recognized as foreign elements, and when they are minified, both case-sensitivity and self-closing slashes are preserved, regardless of the minification settings used for the rest of the file. This ensures valid output for these namespaced elements.

Working with invalid or partial markup

By default, HTML Minifier Next parses markup into a complete tree structure, then modifies it (removing anything that was specified for removal, ignoring anything that was specified to be ignored, etc.), then creates markup from that tree and returns it.

Input markup (e.g., <p id="">foo) → Internal representation of markup in a form of tree (e.g., { tag: "p", attr: "id", children: ["foo"] }) → Transformation of internal representation (e.g., removal of id attribute) → Output of resulting markup (e.g., <p>foo</p>)

For partial HTML fragments (such as template includes, SSI fragments, or closing tags without opening tags), use the partialMarkup: true option. This preserves stray end tags (closing tags without corresponding opening tags) and prevents auto-closing of unclosed tags at the end of input. Note that normal HTML auto-closing rules still apply during parsing—for example, a closing parent tag will still auto-close its unclosed child elements.

To validate complete HTML markup, use the W3C validator or one of several validator packages.

Security

ReDoS protection

This minifier includes protection against regular expression denial of service (ReDoS) attacks:

• Custom fragment quantifier limits: The customFragmentQuantifierLimit option (default: 200) prevents exponential backtracking by replacing unlimited quantifiers (*, +) with bounded ones in regular expressions.

• Input length limits: The maxInputLength option allows you to set a maximum input size to prevent processing of excessively large inputs that could cause performance issues.

• Enhanced pattern detection: The minifier detects and warns about various ReDoS-prone patterns including nested quantifiers, alternation with quantifiers, and multiple unlimited quantifiers.

Important: When using custom ignoreCustomFragments, ensure your regular expressions don’t contain unlimited quantifiers (*, +) without bounds, as these can lead to ReDoS vulnerabilities.

Custom fragment examples

Safe patterns (recommended):

ignoreCustomFragments: [
  /<%[\s\S]{0,1000}?%>/,         // JSP/ASP with explicit bounds
  /<\?php[\s\S]{0,5000}?\?>/,    // PHP with bounds
  /\{\{[^}]{0,500}\}\}/          // Handlebars without nested braces
]

Potentially unsafe patterns (will trigger warnings):

ignoreCustomFragments: [
  /<%[\s\S]*?%>/,                // Unlimited quantifiers
  /<!--[\s\S]*?-->/,             // Could cause issues with very long comments
  /\{\{.*?\}\}/,                 // Nested unlimited quantifiers
  /(script|style)[\s\S]*?/       // Multiple unlimited quantifiers
]

Template engine configurations:

// Handlebars/Mustache
ignoreCustomFragments: [/\{\{[\s\S]{0,1000}?\}\}/]

// Liquid (Jekyll)
ignoreCustomFragments: [/\{%[\s\S]{0,500}?%\}/, /\{\{[\s\S]{0,500}?\}\}/]

// Angular
ignoreCustomFragments: [/\{\{[\s\S]{0,500}?\}\}/]

// Vue.js
ignoreCustomFragments: [/\{\{[\s\S]{0,500}?\}\}/]

Important: When using custom ignoreCustomFragments, the minifier automatically applies bounded quantifiers to prevent ReDoS attacks, but you can also write safer patterns yourself using explicit bounds.

Escaping patterns in different contexts

The escaping requirements for ignoreCustomFragments patterns differ depending on how you’re using HMN:

Config file (JSON):

{
  "ignoreCustomFragments": ["\\{%[\\s\\S]{0,1000}?%\\}", "\\{\\{[\\s\\S]{0,500}?\\}\\}"]
}

Programmatic (JavaScript/Node.js):

ignoreCustomFragments: [/\{%[\s\S]{0,1000}?%\}/, /\{\{[\s\S]{0,500}?\}\}/]

CLI (via config file—recommended):

html-minifier-next --config-file=config.json input.html

CLI (inline—not recommended due to complex escaping):

html-minifier-next --ignore-custom-fragments '[\\\"\\\\{%[\\\\s\\\\S]{0,1000}?%\\\\}\\\"]' input.html

For CLI usage, using a config file is strongly recommended to avoid complex shell and JSON escaping.

Web demo:

\{%[\s\S]{0,1000}?%\} \{\{[\s\S]{0,500}?\}\}

Running HTML Minifier Next locally

Local server

Regression tests

cd backtest;
npm i;
npm run backtest

The backtest tool tracks minification performance across Git history. Results are saved in the backtest folder as CSV and JSON files.

Parameters:

• No argument: Tests last 50 commits (default)
• COUNT: Tests last COUNT commits (e.g., npm run backtest 100)
• COUNT/STEP: Tests last COUNT commits, sampling every STEPth commit (e.g., npm run backtest 500/10 tests 50 commits)

Acknowledgements

With many thanks to all the previous authors of HTML Minifier, especially Juriy “kangax” Zaytsev, and to everyone who helped make this new edition better, particularly Daniel Ruf and Jonas Geiler.