Lint your HTML by adding a stylesheet to your project. Errors will get an outline style (warning or error). A hint will tell what can be done to fix the issue.
Insert the distributed css file to your project. Preferrable at the end of your document root.
Another option is to use an extension like User CSS to append the css afterwards in the browser.
Open the Firefox Developer Tools and click the Style Editor Tab. Click the Import-Button and select the distributed css file from your disk to apply it.
Developers can inspect the warning/error by reading the message in the devtools styles (filter styles for "content").
- Authors are listed in each css file
- Webpack Boilerplate from Tania Rascia