because it violates the following Content Security Policy directive: "style-src getfirebug.com 'self' 'unsafe-inline'". Note that 'style-src-elem' was not explicitly set, so 'style-src' is used as a fallback. code example

Example: because it violates the following Content Security Policy directive: "default-src 'none'". Note that 'font-src' was not explicitly set, so 'default-src' is used as a fallback

devServer: {

        historyApiFallback: true
    }
};