i need use less-loader,but i found that the postcss config is fixed,and I could only use CSS configuration.
file: node_modules/microbundle-crl/src/index.js

Hiya - this is not webpack, so less-loader is not available. However, less and sass are both supported by default, you simply need to install the relevant processor:
npm i -D less
Now you can import .less files.
I'm going to close this out as wontfix since Microbundle is specifically not trying to be a general-purpose configurable bundler, and this falls into that territory. Please let me know though if the above doesn't work.
Hello @developit , I tried with your approach but stuck with an error like below:
$ microbundle --no-compress --format modern,cjs
(postcss plugin) Error: Inline JavaScript is not enabled. Is it set in your options?
I understood that I need to set javascriptEnabled: true as what we usually do for webpack less-loader, but I can't find how to handle it with microbundle, can shed some light on this? Thank you very much!
You'd have to find a dotfile used by rollup-plugin-postcss. Microbundle does not allow customization of CSS beyond filenames.
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I'm going to close this out as wontfix since Microbundle is specifically not trying to be a general-purpose configurable bundler, and this falls into that territory. Please let me know though if the above doesn't work.