BundleTransformer.AutoPrefixer Not Prefixing

Question

I have a .net mvc site, which we have configured to use the BundleTransformer nuget modute, and this is working well for us for a while now. We now have some new sass coming down the pipe that is going to require vendor specific prefixes, so I'm trying to get the Autoprefixer module working in our solution, and things are not going well. No matter what I seem to try, it just doesn't seem to do anything.

First thing I did was go into into nuget, and installed BundleTransformer.Autoprefixer and JavaScriptEngineSwitcher.Msie packages.

Next, I added some parts to the web.config:

In configuration/configSections/SectionGroup name="bundleTransformer"

<section name="autoprefixer" type="BundleTransformer.Autoprefixer.Configuration.AutoprefixerSettings, BundleTransformer.Autoprefixer" />

In configuration/bundleTransformer/core/css/postProcessors

<add name="AutoprefixCssPostProcessor" type="BundleTransformer.Autoprefixer.PostProcessors.AutoprefixCssPostProcessor, BundleTransformer.Autoprefixer" useInDebugMode="false" />

In configuration/bundleTransformer

<autoprefixer>
  <jsEngine name="MsieJsEngine" />
</autoprefixer>

Lastly, in my code where I set up my bundles, I have added this:

JsEngineSwitcher engineSwitcher = JsEngineSwitcher.Instance;
engineSwitcher.EngineFactories
  .AddMsie(new MsieSettings
  {
    UseEcmaScript5Polyfill = true,
    UseJson2Library = true,
    EngineMode = JsEngineMode.ChakraIeJsRt
  });

engineSwitcher.DefaultEngineName = MsieJsEngine.EngineName;

bundles.UseCdn = true;
BundleResolver.Current = new CustomBundleResolver();

I tried this with and without setting the EngineMode setting. All the examples I've found don't have it, but the instructions here explicitly say that the msie engine only works in the chakra jsrt mode... so I figured it was worth a shot. Didn't seem to do anything.

End result is that my sass and JS is still bundling and minifying like it has been, but there is no vendor prefixes. Normally when something goes wrong with the bundling it crashes pretty hard, but in this case it seems to just... not do anything. No errors, nothing in the logs that I can see.

Does anyone know if I've missed something, or something is configured wrong. Or even some direction on how to better debug what's going on?