csslex

const cssText = `
	/* comment **/
	@import url('style.css') print;
	body {
	  background-color: white;
	  color: #222
	}
	div p,
	  #id:first-line {
	    white-space: nowrap; }
	@media print {
	  body { font-size: 10pt }
	}
	.c1{color:red}.c2{color:blue}
	`
style := make(map[string]map[string]string)
var skip bool
var sel []string
for item := range Lex(cssText) {
	switch item.Typ {
	case ItemError:
		log.Fatal(item)
	case ItemAtRuleIdent, ItemAtRule:
		continue
	case ItemAtRuleBlockStart:
		skip = true
		continue
	case ItemAtRuleBlockEnd:
		skip = false
		continue
	case ItemBlockEnd:
		sel = nil
	case ItemSelector:
		if skip {
			continue
		}
		sel = append(sel, item.Val)
		if _, ok := style[item.Val]; !ok {
			style[item.Val] = make(map[string]string)
		}
	case ItemDecl:
		if skip || len(sel) == 0 {
			continue
		}
		decl := strings.SplitN(item.Val, ":", 2)
		decl[0] = strings.TrimSpace(decl[0])
		decl[1] = strings.TrimSpace(decl[1])
		for _, s := range sel {
			style[s][decl[0]] = decl[1]
		}
	}
}
sb, err := json.MarshalIndent(style, "", "  ")
if err != nil {
	log.Fatal(err)
}
fmt.Println(string(sb))

Output:

{
  "#id:first-line": {
    "white-space": "nowrap"
  },
  ".c1": {
    "color": "red"
  },
  ".c2": {
    "color": "blue"
  },
  "body": {
    "background-color": "white",
    "color": "#222"
  },
  "div p": {
    "white-space": "nowrap"
  }
}