html-minifier-next

HTML Minifier Next

HTML Minifier Next (HMN) is a super-configurable, well-tested, JavaScript-based HTML minifier.

The project was based on HTML Minifier Terser, which in turn had been based on Juriy “kangax” Zaytsev’s HTML Minifier. HMN offers additional features, but is backwards-compatible with both. The project was set up because as of 2025, both HTML Minifier Terser and HTML Minifier had been unmaintained for a few years. As the project seems maintainable [to me, Jens, an HTML optimizer]—even more so with community support—, it’s being updated, extended, and documented further in this place.

Installation

From npm for use as a command line app:

npm i -g html-minifier-next

From npm for programmatic use:

npm i html-minifier-next

General usage

CLI

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

Option	Description	Example
--input-dir <dir>	Specify an input directory (best restricted with --file-ext)	--input-dir=src
--ignore-dir <patterns>	Exclude directories—relative to input directory—from processing (comma-separated, overrides config file setting)	--ignore-dir=libs, --ignore-dir=libs,vendor,node_modules
--output-dir <dir>	Specify an output directory	--output-dir=dist
--file-ext <extensions>	Specify file extension(s) to process (comma-separated, overrides config file setting)	--file-ext=html, --file-ext=html,htm,php, --file-ext="html, htm, php"
-o <file>, --output <file>	Specify output file (reads from file arguments or STDIN)	File to file: html-minifier-next input.html -o output.html Pipe to file: cat input.html | html-minifier-next -o output.html File to STDOUT: html-minifier-next input.html
-c <file>, --config-file <file>	Use a configuration file	--config-file=html-minifier.json
--preset <name>	Use a preset configuration (conservative or comprehensive)	--preset=conservative
-v, --verbose	Show detailed processing information (active options, file statistics)	html-minifier-next --input-dir=src --output-dir=dist --verbose --collapse-whitespace
-d, --dry	Dry run: Process and report statistics without writing output	html-minifier-next input.html --dry --collapse-whitespace
-V, --version	Output the version number	html-minifier-next --version

Configuration file

You can also 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,htm",
  "ignoreDir": "libs,vendor"
}

JavaScript module configuration example:

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

Using a configuration file:

# Specify config file
html-minifier-next --config-file=html-minifier.json --input-dir=src --output-dir=dist

# CLI arguments override config file settings
html-minifier-next --config-file=html-minifier.json --file-ext=xml --input-dir=src --output-dir=dist

Node.js

ESM with Node.js ≥16.14:

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

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

CommonJS:

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

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

See the original blog post for details of how it works, description of each option, testing results, and conclusions.

For lint-like capabilities, take a look at HTMLLint.

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: Safe minification suitable for most projects. Includes whitespace collapsing, comment removal, and doctype normalization.
• comprehensive: Aggressive minification for maximum file size reduction. Includes 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.

Option (config/CLI)	Description	Default
caseSensitive --case-sensitive	Treat attributes in case-sensitive manner (useful for custom HTML elements)	false
collapseAttributeWhitespace --collapse-attribute-whitespace	Trim and collapse whitespace characters within attribute values	false
collapseBooleanAttributes --collapse-boolean-attributes	Omit attribute values from boolean attributes	false
collapseInlineTagWhitespace --collapse-inline-tag-whitespace	Don’t leave any spaces between display: inline; elements when collapsing—use with collapseWhitespace: true	false
collapseWhitespace --collapse-whitespace	Collapse whitespace that contributes to text nodes in a document tree	false
conservativeCollapse --conservative-collapse	Always collapse to one space (never remove it entirely)—use with collapseWhitespace: true	false
continueOnMinifyError --no-continue-on-minify-error	Continue on minification errors; when false, minification errors throw and abort processing	true
continueOnParseError --continue-on-parse-error	Handle parse errors instead of aborting	false
customAttrAssign --custom-attr-assign	Arrays of regexes that allow to support custom attribute assign expressions (e.g., <div flex?="{{mode != cover}}"></div>)	[]
customAttrCollapse --custom-attr-collapse	Regex that specifies custom attribute to strip newlines from (e.g., /ng-class/)
customAttrSurround --custom-attr-surround	Arrays of regexes that allow to support custom attribute surround expressions (e.g., <input {{#if value}}checked="checked"{{/if}}>)	[]
customEventAttributes --custom-event-attributes	Arrays of regexes that allow to support custom event attributes for minifyJS (e.g., ng-click)	[ /^on[a-z]{3,}$/ ]
customFragmentQuantifierLimit --custom-fragment-quantifier-limit	Set maximum quantifier limit for custom fragments to prevent ReDoS attacks	200
decodeEntities --decode-entities	Use direct Unicode characters whenever possible	false
html5 --no-html5	Parse input according to the HTML specification; when false, enforces legacy inline/block nesting rules that may restructure modern HTML	true
ignoreCustomComments --ignore-custom-comments	Array of regexes that allow to ignore certain comments, when matched	[ /^!/, /^\s*#/ ]
ignoreCustomFragments --ignore-custom-fragments	Array of regexes that allow to ignore certain fragments, when matched (e.g., <?php … ?>, {{ … }}, etc.)	[ /<%[\s\S]*?%>/, /<\?[\s\S]*?\?>/ ]
includeAutoGeneratedTags --no-include-auto-generated-tags	Insert elements generated by HTML parser; when false, omits auto-generated tags	true
inlineCustomElements --inline-custom-elements	Array of names of custom elements which are inline	[]
keepClosingSlash --keep-closing-slash	Keep the trailing slash on void elements	false
maxInputLength --max-input-length	Maximum input length to prevent ReDoS attacks (disabled by default)	undefined
maxLineLength --max-line-length	Specify a maximum line length; compressed output will be split by newlines at valid HTML split-points
minifyCSS --minify-css	Minify CSS in style elements and style attributes (uses Lightning CSS)	false (could be true, Object, Function(text, type))
minifyJS --minify-js	Minify JavaScript in script elements and event attributes (uses Terser)	false (could be true, Object, Function(text, inline))
minifyURLs --minify-urls	Minify URLs in various attributes (uses relateurl)	false (could be String, Object, Function(text), async Function(text))
noNewlinesBeforeTagClose --no-newlines-before-tag-close	Never add a newline before a tag that closes an element	false
partialMarkup --partial-markup	Treat input as a partial HTML fragment, preserving stray end tags (closing tags without opening tags) and preventing auto-closing of unclosed tags at end of input	false
preserveLineBreaks --preserve-line-breaks	Always collapse to one line break (never remove it entirely) when whitespace between tags includes a line break—use with collapseWhitespace: true	false
preventAttributesEscaping --prevent-attributes-escaping	Prevents the escaping of the values of attributes	false
processConditionalComments --process-conditional-comments	Process contents of conditional comments through minifier	false
processScripts --process-scripts	Array of strings corresponding to types of script elements to process through minifier (e.g., text/ng-template, text/x-handlebars-template, etc.)	[]
quoteCharacter --quote-character	Type of quote to use for attribute values (' or ")	Auto-detected (uses the quote requiring less escaping; defaults to " when equal)
removeAttributeQuotes --remove-attribute-quotes	Remove quotes around attributes when possible	false
removeComments --remove-comments	Strip HTML comments	false
removeEmptyAttributes --remove-empty-attributes	Remove all attributes with whitespace-only values	false (could be true, Function(attrName, tag))
removeEmptyElements --remove-empty-elements	Remove all elements with empty contents	false
removeEmptyElementsExcept --remove-empty-elements-except	Array of elements to preserve when removeEmptyElements is enabled; accepts simple tag names (e.g., ["td"]) or HTML-like markup with attributes (e.g., ["<span aria-hidden='true'>"]); supports double quotes, single quotes, and unquoted attribute values	[]
removeOptionalTags --remove-optional-tags	Remove optional tags	false
removeRedundantAttributes --remove-redundant-attributes	Remove attributes when value matches default	false
removeScriptTypeAttributes --remove-script-type-attributes	Remove type="text/javascript" from script elements; other type attribute values are left intact	false
removeStyleLinkTypeAttributes --remove-style-link-type-attributes	Remove type="text/css" from style and link elements; other type attribute values are left intact	false
removeTagWhitespace --remove-tag-whitespace	Remove space between attributes whenever possible; note that this will result in invalid HTML	false
sortAttributes --sort-attributes	Sort attributes by frequency	false
sortClassName --sort-class-name	Sort style classes by frequency	false
trimCustomFragments --trim-custom-fragments	Trim whitespace around ignoreCustomFragments	false
useShortDoctype --use-short-doctype	Replaces the doctype with the short (HTML) doctype	false

Sorting attributes and style classes

Minifier options like sortAttributes and sortClassName 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 with Lightning CSS

When minifyCSS is set to true, HTML Minifier Next uses Lightning CSS to minify CSS in <style> elements and style 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);
  }
});

Minification comparison

How does HTML Minifier Next compare to other minifiers? (All with the most aggressive settings, though without hyper-optimization. Minimize does not minify CSS or JS.)

Site	Original Size (KB)	HTML Minifier Next	HTML Minifier Terser	htmlnano	@swc/html	minify-html	minimize	html­com­pressor.­com
A List Apart	59	49	50	51	52	51	54	52
Apple	266	207	207	236	239	240	242	243
BBC	739	675	685	695	695	697	733	n/a
CERN	156	87	87	95	95	95	97	100
CSS-Tricks	161	121	119	127	142	142	147	144
ECMAScript	7238	6341	6341	6561	6444	6567	6614	n/a
EDRi	80	59	60	70	70	71	75	73
EFF	55	47	47	50	48	49	50	50
European Alternatives	48	30	30	32	32	32	32	32
FAZ	1543	1436	1441	1383	1468	1479	1490	n/a
French Tech	152	121	122	125	125	125	132	126
Frontend Dogma	223	213	215	236	221	223	241	222
Google	18	17	17	17	17	17	18	18
Ground News	1437	1222	1225	1320	1346	1353	1424	n/a
HTML Living Standard	149	147	147	153	147	149	155	149
Igalia	50	34	34	36	36	36	37	37
Leanpub	1288	1092	1092	1099	1097	1094	1283	n/a
Mastodon	37	27	27	32	34	35	35	35
MDN	109	62	62	64	65	65	68	68
Middle East Eye	223	196	196	203	201	200	202	203
Nielsen Norman Group	86	74	74	55	74	75	77	76
SitePoint	485	354	354	425	459	464	481	n/a
TetraLogical	44	38	38	35	38	38	39	39
TPGi	175	160	161	160	164	166	172	172
United Nations	150	112	113	120	124	124	129	122
W3C	50	35	36	38	38	38	40	38
Average processing time		306 ms (26/26)	364 ms (26/26)	162 ms (26/26)	55 ms (26/26)	16 ms (26/26)	316 ms (26/26)	1439 ms (20/26)

(Last updated: Dec 17, 2025)

Examples

CLI

Sample command line:

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

Process specific files and directories:

# Process only HTML files
html-minifier-next --collapse-whitespace --input-dir=src --output-dir=dist --file-ext=html

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

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

# Process all files (default behavior)
html-minifier-next --collapse-whitespace --input-dir=src --output-dir=dist
# When processing all files, non-HTML files will also be read as UTF‑8 and passed to the minifier
# Consider restricting with `--file-ext` to avoid touching binaries (e.g., images, archives)

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: Options: collapseWhitespace, html5, includeAutoGeneratedTags
#   ✓ 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.

Running HTML Minifier Next locally

Local server

npm run serve

Benchmarks

Benchmarks for minified HTML:

cd benchmarks;
npm i;
npm run benchmarks

(In case of dependency conflicts, run npm i with the --legacy-peer-deps flag.)

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.