How to use cssnano as the final CSS minifier with injectStyles: true while keeping Lightning CSS prefixing/transforms?

[zalishchuk]

I am using Rsbuild and I have a case where I must keep output.injectStyles: true (so CSS is injected into JS, not emitted as separate .css files).

Trying to understand the best/supported way to do this:

• keep Lightning CSS for prefixing / CSS transforms
• but use cssnano as the final CSS minifier

@rsbuild/plugin-css-minimizer seems to work at the CSS asset minimizer stage.

With output.injectStyles: true, there are no emitted CSS assets, so this plugin does not appear to be the right tool for this case.

I tried using cssnano in PostCSS, while keeping lightningcss-loader enabled and disabling its minification (minify: false).

In that setup, Lightning CSS still runs after PostCSS (loader order), and it reserializes the CSS. In practice, this reintroduces formatting/newlines/whitespace and cancels out cssnano's minified formatting.

I tested this on my codebase and saw:

• postcss (autoprefixer + cssnano) with Lightning disabled: compact/minified embedded CSS strings
• postcss (autoprefixer + cssnano) + Lightning loader (minify: false): multiline formatted CSS strings added back into the JS bundle, and bundle size increased

So cssnano is not the final serializer if Lightning loader is still enabled afterward.

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Is there a supported way in Rsbuild to keep Lightning CSS prefixing/transforms but still make cssnano the final CSS minifier/serializer when output.injectStyles: true?

I am investigating this because I want predictable final minification behavior, and I am cautious about optimizer rewrites in some CSS cases.

In particular, Lightning CSS can rewrite translate3d(0, 0, 0) to translate(0, 0), which is a bit off for my use case because it can affect compositing behavior (for example in Safari).

One reason I am cautious about Lightning CSS minification right now is this issue/PR context:

• fix(minify): preserve 3d transform semantics during minification parcel-bundler/lightningcss#1157

I do not like that (at least from what I can see in Lightning CSS right now), I cannot easily configure Lightning CSS minifier behavior in a granular way to disable specific optimizations/rewrites that I consider risky for my project.

Thanks.