Without `from` option PostCSS could generate wrong source map or do not find Browserslist config

Finally, this warning solved. Thanks to Suraj Rao

First find sass.js file under /node_modules/@ionic/app-scripts/dist/sass.js. inside search var postcssOptions and add from: undefined,

Finally should be like this:

var postcssOptions = {
            from: undefined,
            to: path_1.basename(sassConfig.outFile),
            map: autoPrefixerMapOptions
        };

Reference： workaround.

For whoever directly used PostCSS and met this issue then came here, Change your

postcss( any_plugins ).process( css )

to

postcss( any_plugins ).process( css, { from: undefined } )

can fix the issue.