HTML Minifier Next

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

The project has been based on Terser’s html-minifier-terser, which in turn had been based on Juriy Zaytsev’s html-minifier (HMN offers additional features, but is compatible with both). It was set up because as of 2025, both html-minifier-terser and html-minifier have been unmaintained for some time. As the project seems maintainable [to me, Jens]—even more so with community support—, it’s being updated 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
--output-dir <dir>	Specify an output directory	--output-dir=dist
--file-ext <extensions>	Specify file extension(s) to process (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
-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"
}

JavaScript module configuration example:

module.exports = {
  collapseWhitespace: true,
  removeComments: true,
  fileExt: "html,htm"
};

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.

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
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 1 space (never remove it entirely)—use with collapseWhitespace: true	false
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	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	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 clean-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
preserveLineBreaks --preserve-line-breaks	Always collapse to 1 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 ")
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
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.

Minification comparison

How does HTML Minifier Next compare to other solutions, like minimize, htmlcompressor.com, htmlnano, and minify-html? (All with the most aggressive settings, but without hyper-optimization.)

Site	Original Size (KB)	HTML Minifier Next	minimize	html­compressor.com	htmlnano	minify-html
A List Apart	62	53	58	56	54	55
Amazon	206	195	203	200	196	n/a
Apple	183	143	169	167	160	166
BBC	689	633	683	n/a	646	648
CSS-Tricks	163	121	149	146	127	145
ECMAScript	7233	6338	6610	n/a	6557	6563
EFF	57	48	52	52	51	50
FAZ	1871	1747	1785	n/a	1647	n/a
Frontend Dogma	119	114	128	119	126	119
Google	18	16	18	18	17	n/a
Ground News	3308	2946	3295	n/a	3042	n/a
HTML	149	147	155	148	153	149
Leanpub	1279	1063	1274	n/a	1070	n/a
Mastodon	35	26	34	34	30	33
MDN	107	61	67	67	63	n/a
Middle East Eye	224	197	204	205	204	202
SitePoint	485	354	481	n/a	423	465
United Nations	151	114	130	123	121	124
W3C	50	36	41	39	38	39

Examples

CLI

Sample command line:

html-minifier-next --collapse-whitespace --remove-comments --minify-js true --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)

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-LD

You can minify script elements with JSON-LD by setting { processScripts: ['application/ld+json'] }. Note that this minification is rudimentary; it’s mainly useful for removing newlines and excessive whitespace.

Preserving SVG elements

SVG elements are automatically recognized, and when they are minified, both case-sensitivity and closing-slashes are preserved, regardless of the minification settings used for the rest of the file.

Working with invalid markup

HTML Minifier Next can’t work with invalid or partial chunks of markup. This is because it parses markup into a tree structure, then modifies it (removing anything that was specified for removal, ignoring anything that was specified to be ignored, etc.), then it creates a markup out of 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>)

HMN can’t know that the original markup represented only part of the tree. It parses a complete tree and, in doing so, loses information about the input being malformed or partial. As a result, it can’t emit a partial or malformed tree.

To validate 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

Benchmarks

Benchmarks for minified HTML:

cd benchmarks
npm install
npm run benchmarks

Local server

npm run serve

Acknowledgements

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