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CustomScriptBundle()
ScriptBundle()

If not you will get the slightly confusing error message:

Could not find the transformer in the '~/Content/css/myCssBundle' bundle.

The HTML is written (just view source) with an additional parameter ?bundleVirtualPath

/Content/css/defenderrazor_DEBUG.less?
bundleVirtualPath=~%2fContent%2fcss%2fmyCssBundle

This is intercepted by the handler in the web.config when a .less file is requested and the handler is then able to use that bundle name to look up any transformations that you've set - including in this case the default post processor for AutoprefixCss.

asp.net - Running Autoprefixer with BundleTransformer / LESS in Debug ...

asp.net less autoprefixer bundletransformer
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For debugging HTTP-handlers to use a configuration settings from bundles need to add in the RegisterBundles method of App_Start/BundleConfig.cs file the following code:

In order to these settings can be applied to CSS- and JS-assets need to register the debugging HTTP-handlers CssAssetHandler and JsAssetHandler in Web.config file. To do this in the IIS Integrated mode, you need add to the /configuration/system.webServer/handlers element the following code:

Web.config
defaultPostProcessors
\configuration\bundleTransformer\core\css
AutoprefixCssPostProcessor
defaultPostProcessors="UrlRewritingCssPostProcessor,AutoprefixCssPostProcessor"

asp.net - Running Autoprefixer with BundleTransformer / LESS in Debug ...

asp.net less autoprefixer bundletransformer