Из комментариев.
Here's an example to explain the difference. Suppose you have the following custom element:
<dom-module id="x-foo">
<template>
<div>I am local dom</div>
<content></content>
</template>
<script>
Polymer({
is: 'x-foo'
});
</script>
</dom-module>
And you use it like this in your document:
<x-foo>I am light dom</x-foo>
What ever you put into the template of the element is local dom. What you put as children to your custom element when you use it is light dom. So, the local dom is determined by the creator the element, while the light dom is set by the user of the element. Of course, when you both create and use your own custom elements you have some flexibility what to put where.
И тут хорошая статья
https://sohabr.net/habr/post/259187/?version=102597 
