Built-in HTML minifier corrupts copy-webpack-plugin .html assets containing {% %} template statements #22013

Have you used AI?

Yes

Which part of webpack is affected?

html

Bug Description

webpack 5.110.0 added a built-in HTML minifier that is on by default in mode: 'production'. It reserializes every .html asset, including files that copy-webpack-plugin passes through unchanged. When such a file contains {% ... %} server-side template statements (the statement delimiter of the Django/Jinja family: Jinja2, Django, Twig, Liquid, Nunjucks, HubSpot HubL), the minifier drops a %} delimiter and emits invalid template markup from valid input.

No HTML loader, html-webpack-plugin, or explicitly configured minimizer is involved. Just mode: 'production' and copy-webpack-plugin copying a .html file.

%} tokenizes as a valid HTML attribute name (the attribute-name state accepts every character except whitespace, /, >, and =), so {% if x %} y {% endif %} produces two %} attribute-name tokens on the same element, and the minifier deduplicates the repeat and loses one %}.

Link to Minimal Reproduction and steps to reproduce

https://github.com/davidding/webpack-5110-html-minify-repro

• npm install
• npm run verify (runs webpack then prints each src/*.html next to its dist/ output)

Expected Behavior

The minifier does not alter content inside {% ... %} in a way that changes it.

If template-awareness is out of scope, two alternatives would each resolve this:

1. HTML minification of assets that webpack only copied through (rather than produced from an HTML-emitting module) is opt-in rather than on-by-default.
2. The 5.110.0 change is documented as breaking, with the migration path and the workarounds called out in release notes.

Actual Behavior

The %} delimiter is dropped, producing invalid markup:

  == src/corrupt-conditional-attribute.html ==
    in : <input type="text"{% if required %} required{% endif %}>
    out: <input type=text {% if required %} required{% endif>

  == src/corrupt-space-inside-conditional.html ==
    in : <div class="btn{% if active %} btn-active{% endif %}"></div>
    out: <div class="btn{% if active %} btn-active{% endif"></div>

  == src/survive-space-outside-conditional.html ==   (control: unchanged)
    in : <div class="btn {% if active %}btn-active{% endif %}"></div>
    out: <div class="btn {% if active %}btn-active{% endif %}"></div>

The type="text" to type=text quote removal is valid minification and is not the problem. The problem is the dropped %}.

Two distinct mechanisms drop the repeated %}:

• Duplicate attribute-name removal (lib/html/syntax.js#L8520-L8528 @ v5.110.3), spec-mandated. Corrupts the conditional-attribute case.
• Token-list deduplication (_normalizeTokenList, lib/html/syntax.js#L10200-L10231 @ v5.110.3), webpack's own class-attribute minify transform, which a browser does not perform. Corrupts the class case.

Environment

System:
  OS: macOS 26.6.2
  CPU: (12) arm64 Apple M3 Pro
  Memory: 18.00 GB
Binaries:
  Node: 24.19.0
  npm: 11.17.0
Packages:
  copy-webpack-plugin: 14.0.0
  webpack: 5.110.3
  webpack-cli: 7.2.3

Is this a regression?

Yes (please specify version below)

Last Working Version

v5.109.2

Additional Context

Byte-identical (correct) output on 5.109.2, the final 5.109.x release, which predates the built-in minifier. Identical corruption on 5.110.0, 5.110.1, 5.110.2, and 5.110.3.

Workarounds (verified against the reproduction on 5.110.3):

1. optimization: { minimize: { html: false } } disables HTML minification only; JS/CSS minification stays on. Schema option added in 5.110.0.
2. info: { minimized: true } on the copy-webpack-plugin pattern marks the copied assets already-minimized so the minifier skips them. Version-agnostic (works pre-5.110 too), so it is the right choice for a shared config that must support both.
3. experiments: { html: false } turns off the built-in HTML pipeline entirely.
4. Pin webpack < 5.110. Stopgap only.