Pull Request resolves build error in PR #47994

• I read the Generative AI policy and my contribution is either not created with the help of AI or is compatible with the policy and GNU/GPL 2 or later.

Summary of Changes

The vendor CSS versioning step (build/build-modules-js/stylesheets/css-versioning.mjs) runs every CSS file in media/vendor through lightningcss, which fully parses and re-serialises the file in order to append cache-busting versions to url(...) references.

The problem: lightningcss does not just rewrite URLs, it semantically parses the whole stylesheet. As soon as a vendor package ships modern CSS syntax that the bundled lightningcss version cannot parse yet, the parser throws and the entire build aborts.

See PR #47994 from @brianteeman https://github.com/joomla/joomla-cms/actions/runs/27872556107/job/82563289058?pr=47994

This PR adds a non-destructive fallback:

1. First try lightningcss as before.
2. If parsing fails, do not abort the build.
3. Instead, only url(...) values are versioned via a text-based rewrite, leaving the rest of the CSS — including any syntax the parser rejected — completely untouched.
4. A log line indicates which file fell back, so the behaviour is transparent.

This is more robust long-term for vendor CSS (TinyMCE, Bootstrap, FontAwesome, …), since those packages can introduce new valid CSS syntax at any time.

Testing Instructions

1. Add a CSS file under media/vendor/ that contains CSS syntax the bundled lightningcss cannot parse, together with at least one local url(...) reference, e.g.:
   .a..{color:red}
   .b{background:url(some-existing-asset.png)}
2. (.a..{} is intentionally unparseable for lightningcss and stands in for not-yet-supported modern vendor syntax.)
3. Run npm ci followed by npm run cssversioning (or the full php build/build.php).

Actual result BEFORE applying this Pull Request

The build aborts with an error such as:

Error: SyntaxError: Unexpected token Ident("of")
      at fixVersion (.../build-modules-js/stylesheets/css-versioning.mjs)

No CSS is versioned and the build fails.

Expected result AFTER applying this Pull Request

The build completes. The unparseable file logs a fallback notice (Could not parse … Falling back to text-based url() versioning.), its url(...) references receive a version query, and all other CSS — including the syntax lightningcss could not parse — is preserved byte-for-byte.

Link to documentations

Please select:

• Documentation link for guide.joomla.org:
• No documentation changes for guide.joomla.org needed
• Pull Request link for manual.joomla.org:
• No documentation changes for manual.joomla.org needed