How browser engine store Css values and using it?

Question

I am trying to write an browser engine from scratch. I wonder how browser engines store css values and using it. I look into blink source code and I think at first there are property-value string-string pairs and then it turns into some structs called ComputedStyle. But I still didn't understand how blink use them. I mean does blink store css values in render object or it does only store those values in cssom. At last why browser engines need both store css values as string, store css values as enumaration(some of them) in cssom, store necessary css values in render object? Isn't it possible these three combined to one?