Diagnostic tools
https://gtmetrix.com/
https://developers.google.com/speed/pagespeed/insights/
https://varvy.com/pagespeed/
CSS & HTML Validation
https://jigsaw.w3.org/css-validator/
https://validator.w3.org/
WordPress Plugins:
https://de.wordpress.org/plugins/w3-total-cache/
https://de.wordpress.org/plugins/autoptimize/
see also:
http://www.onlinemediamasters.com/w3-total-cache-settings/