Web Component problem

### Existing Component
Yes

### Component Name
All components from Element UI are affected

### Description
When building Vue app as Web Component I have a problem where styles from _ElementUI_ lib are not loaded and _ElementUI_ components are not visible.
As a workaround, I introduced **shadow-dom** and added styles manually but in such scenario, **popper.js** throws the following exception when it comes to the method `getStyleComputedProperty`


as shadow root is not of type Element.

as a possible fix, element type check or element parent check (as shadow-dom doesn't have a parent) could be introduced