HTML Minifier Next

HTML Minifier 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. 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 will be 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

Usage

Note that almost all options are disabled by default. Experiment and find what works best for you and your project.

Sample command line:

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

Process specific file extensions:

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

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

# Using configuration file with fileExt setting
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

CLI Options

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

Option	Description	Example
--input-dir <dir>	Specify an input directory	--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 or --file-ext=html,htm,php
-o --output <file>	Specify output file (single file mode)	-o minified.html
-c --config-file <file>	Use a configuration file	--config-file=html-minifier.json

Configuration Files

You can also use a configuration file to specify options:

JSON configuration example:

{
  "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

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

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

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.

Minification comparison

How does HTML Minifier compare to other solutions, like minimize or htmlcompressor.com?

Site	Original size (KB)	HTML Minifier	minimize	htmlcompressor.com
A List Apart	64	54	59	57
Amazon	206	195	203	200
BBC	767	703	761	n/a
CSS-Tricks	166	124	152	148
ECMAScript	7204	6361	6581	n/a
EFF	57	48	52	52
FAZ	1767	1641	1679	n/a
Frontend Dogma	119	114	128	118
Google	51	46	50	50
HTML Minifier	373	250	349	n/a
Mastodon	37	28	36	36
NBC	601	549	593	n/a
New York Times	822	701	811	n/a
United Nations	9	7	8	8
W3C	50	36	41	39
Wikipedia	225	204	215	215

Options quick reference

Most of the options are disabled by default.

Option	Description	Default
caseSensitive	Treat attributes in case-sensitive manner (useful for custom HTML elements)	false
collapseBooleanAttributes	Omit attribute values from boolean attributes	false
customFragmentQuantifierLimit	Set maximum quantifier limit for custom fragments to prevent ReDoS attacks	200
collapseInlineTagWhitespace	Don’t leave any spaces between display:inline; elements when collapsing. Must be used in conjunction with collapseWhitespace=true	false
collapseWhitespace	Collapse white space that contributes to text nodes in a document tree	false
conservativeCollapse	Always collapse to 1 space (never remove it entirely). Must be used in conjunction with collapseWhitespace=true	false
continueOnParseError	Handle parse errors instead of aborting.	false
customAttrAssign	Arrays of regex’es that allow to support custom attribute assign expressions (e.g. '<div flex?="{{mode != cover}}"></div>')	[]
customAttrCollapse	Regex that specifies custom attribute to strip newlines from (e.g. /ng-class/)
customAttrSurround	Arrays of regexes that allow to support custom attribute surround expressions (e.g. <input {{#if value}}checked="checked"{{/if}}>)	[]
customEventAttributes	Arrays of regexes that allow to support custom event attributes for minifyJS (e.g. ng-click)	[ /^on[a-z]{3,}$/ ]
decodeEntities	Use direct Unicode characters whenever possible	false
fileExt	File extensions to process	[] (process all files)
html5	Parse input according to HTML5 specifications	true
ignoreCustomComments	Array of regexes that allow to ignore certain comments, when matched	[ /^!/, /^\s*#/ ]
ignoreCustomFragments	Array of regexes that allow to ignore certain fragments, when matched (e.g. <?php ... ?>, {{ ... }}, etc.)	[ /<%[\s\S]*?%>/, /<\?[\s\S]*?\?>/ ]
includeAutoGeneratedTags	Insert elements generated by HTML parser	true
inlineCustomElements	Array of names of custom elements which are inline	[]
keepClosingSlash	Keep the trailing slash on singleton elements	false
maxInputLength	Maximum input length to prevent ReDoS attacks (disabled by default)	undefined
maxLineLength	Specify a maximum line length. Compressed output will be split by newlines at valid HTML split-points
minifyCSS	Minify CSS in style elements and style attributes (uses clean-css)	false (could be true, Object, Function(text, type))
minifyJS	Minify JavaScript in script elements and event attributes (uses Terser)	false (could be true, Object, Function(text, inline))
minifyURLs	Minify URLs in various attributes (uses relateurl)	false (could be String, Object, Function(text))
noNewlinesBeforeTagClose	Never add a newline before a tag that closes an element	false
preserveLineBreaks	Always collapse to 1 line break (never remove it entirely) when whitespace between tags include a line break. Must be used in conjunction with collapseWhitespace=true	false
preventAttributesEscaping	Prevents the escaping of the values of attributes	false
processConditionalComments	Process contents of conditional comments through minifier	false
processScripts	Array of strings corresponding to types of script elements to process through minifier (e.g. text/ng-template, text/x-handlebars-template, etc.)	[]
quoteCharacter	Type of quote to use for attribute values (“'” or “"”)
removeAttributeQuotes	Remove quotes around attributes when possible	false
removeComments	Strip HTML comments	false
removeEmptyAttributes	Remove all attributes with whitespace-only values	false (could be true, Function(attrName, tag))
removeEmptyElements	Remove all elements with empty contents	false
removeOptionalTags	Remove optional tags	false
removeRedundantAttributes	Remove attributes when value matches default.	false
removeScriptTypeAttributes	Remove type="text/javascript" from script elements. Other type attribute values are left intact	false
removeStyleLinkTypeAttributes	Remove type="text/css" from style and link elements. Other type attribute values are left intact	false
removeTagWhitespace	Remove space between attributes whenever possible. Note that this will result in invalid HTML!	false
sortAttributes	Sort attributes by frequency	false
sortClassName	Sort style classes by frequency	false
trimCustomFragments	Trim white space around ignoreCustomFragments.	false
useShortDoctype	Replaces the doctype with the short (HTML5) doctype	false

Sorting attributes / style classes

Minifier options like sortAttributes and sortClassName won’t impact the plain-text size of the output. However, they form long repetitive chains of characters that should improve compression ratio of gzip used in HTTP compression.

Special cases

Ignoring chunks of markup

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

Minifying JSON-LD

You can minify script elements with JSON-LD by setting the option { processScripts: ['application/ld+json'] }. Note that this minification is very rudimentary, it is 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 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>)

HTML Minifier can’t know that original markup was only half of the tree; it does its best to try to parse it as a full tree and it loses information about tree being malformed or partial in the beginning. As a result, it can’t create a partial/malformed tree at the time of the output.

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.

(Further improvements are needed. Contributions welcome.)

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 benchmarks

Benchmarks for minified HTML:

cd benchmarks
npm install
npm run benchmark

Running 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, like Daniel Ruf.