Why is my style.css page showing a warning in Pingdom speed test?


I am trying to figure out why my website is loading very slowly. I have tried all the usual things like installing W3 Cache, Image Optimisation, etc.

I notice on the Pingdom Speed Test that my style.css sheet has a warning next to it. Does anyone have any idea what this means? (I have attached a screenshot)

Also, I was told that when someone opened the page, they received a HTML only version. I checked it on many different computers and they were all fine, however I wonder if this is related. Is it possible that improving this could improve the reliability?