find a way to preserve px value in some case(especially :root)

lib/stylus-px2rem/mixins.styl

@@ -10,6 +10,8 @@ html-font-size ?= 16px;
 style-names ?= "min-height" "max-height" "min-width" "max-width" "width" "height" "border" "margin" "margin-top" "margin-bottom" "margin-left" "margin-right" "padding" "padding-left" "padding-right" "padding-bottom" "padding-top" "line-height"
 // value less then ignore_limit will be ignored and return the original value
 px2rem_ignore_limit ?= 1
+// if you want to preserve px value in some case(html element for example), use `fontsize '16px!'`, and also you can override preserve_px to '_px'(then `fontsize '16_px'`) or something else
+preserve_px ?= 'px!'
 
 px2rem(prop,values){
     values-px = null;
@@ -26,9 +28,11 @@ px2rem(prop,values){
                 value-rem = unit(value ,'rem');
             }
             push(values-rem,value-rem)
-        } else {
+        } else if(match(preserve_px+'$',''+value)) {
+            push(values-rem,unquote(replace(preserve_px,px,value)))
+        }  else {
             push(values-rem,value)
         }
     }
     {prop} : values-rem;
-}
+}