Use custom variable in html.tpl.php for critical css
Instructions for the Critical CSS module:

Navigate to: admin/config/development/performance/advagg/critical-css

Under Add Critical CSS
- Select the theme that is your front page; usually the default is correct.
- User type should be set to 'anonymous' under most circumstances.
- Type of lookup, select URL
- Value to lookup, type in 
- Critical CSS, paste in the generated CSS from running your homepage url
through https://www.sitelocity.com/critical-path-css-generator which is inside
of the 'Critical Path CSS' textarea on the sitelocity page.
- Click Save Configuration.