It’d be neat to have this: http://www.w3schools.com/css/css_outline.asp in the Designer.
If I’m correct, Outline has little benefits because it doesn’t count in the element dimensions. So its only real avantage against shadows is that it can be dotted or dashed, or have the other border properties (inset etc). If you don’t need those options, you can do the same with inner and outer shadows.
Check here, made with Webflow: http://sab.webflow.io/outline