A critical CSS file MUST be generated in advance for any bundle or entity that you want to have this functionality. Using <a href="@critical_url" target="_blank"><em>critical</em> from Addy Osmani</a> or <a href="@criticalCSS_url" target="_blank"><em>criticalCSS</em> from Filament Group</a> is highly recommended. There are also some <a href="@online_generators_url" target="_blank">Critical CSS online generators</a> to get it without effort, or you could generate it manually, possibly using <a href="@chrome_css_coverage_url">Chrome's CSS Coverage</a>.