autoprefixer incorrectly expands text-decoration with redundant vendor-prefixed and duplicate declarations

[lukhanina]

Bug Description
When processing CSS, autoprefixer converts a clean text-decoration shorthand into multiple redundant declarations, including unnecessary -webkit-text-decoration lines and duplicated text-decoration: underline. This bloats the output, obscures intent, and provides no compatibility benefit in modern browsers.

Input CSS

abbr[title] {
  border-bottom: none; /* 1 */
  text-decoration: underline; /* 2 */
  text-decoration: underline dotted; /* 2 */
}

Output CSS (Actual)

abbr[title] {
  border-bottom: none; /* 1 */
  -webkit-text-decoration: underline;
  text-decoration: underline; /* 2 */
  text-decoration: underline;
  -webkit-text-decoration: underline dotted;
          text-decoration: underline dotted; /* 2 */
}

Impact
Increases bundle size and reduces readability.
Misleads about required browser support.
Obscures intended progressive enhancement and makes diffs/noise in code review.

Link to Minimal Reproduction and step to reproduce
Configure a project with postcss-preset-env (default setup; no legacy prefixes needed)*.

abbr[title] {
  border-bottom: none; /* 1 */
  text-decoration: underline; /* 2 */
  text-decoration: underline dotted; /* 2 */
}

Add the input CSS (above) to a stylesheet.
Build the project and inspect the processed CSS output.
Observe redundant prefixed and duplicated declarations in the abbr[title] block

Expected Behavior
autoprefixer should preserve a minimal, standards-compliant output:

abbr[title] {
	border-bottom: none; /* 1 */
	text-decoration: underline; /* 2 */
	-webkit-text-decoration: underline dotted;
	        text-decoration: underline dotted; /* 2 */
}

No vendor-prefixed -webkit-text-decoration, as modern browsers support the unprefixed shorthand.
No duplicated text-decoration: underline;.
Progressive enhancement remains clear: fallback underline → dotted underline where supported.

Actual Behavior
Adds -webkit-text-decoration lines (obsolete for modern engines).
Duplicates text-decoration: underline;.
Produces noisy CSS that complicates reviews and maintenance without improving compatibility.

Environment
Tooling: autoprefixer (10.4.23) from default or typical PostCSS plugins (postcss-preset-env version 11.1.2).
Browsers: Chrome/Edge (Chromium), Safari, Firefox (modern versions).
OS: Any.
Webpack: 5.96.1

Is this a regression?
None

[ai]

Good idea of improvement.

I am busy on another project next year and, honestly, is not very motivated with this idea.

But if you will prepare PR I will help you and release it.

[lukhanina]

@ai Thank you for the answer
I prepared a commit here csstools/postcss-plugins#1775
it fixes duplication in edge cases -

1. -webkit-text-decoration for non shorthand values will not be added in case next value is also text-decoration, but we have comments in between
2. text-decoration after a shorthand handling will not be added in case it's already presented with the same value above

Kindly ask you or someone else responsible to take a look

[ai]

Closing since it was fixed in postcss-preset-env