custom-style vs shared-styles in polymer
<p>Polymer has support for <code>&lt;style is="custom-style"&gt;</code> which allows you to define styles that only apply to elements, e.g. the shadow DOM.</p>

<p>Polymer also has support for <code>&lt;dom-module id="shared-styles"&gt;</code> which allows you to package a set of style declarations that can be imported into an element definition. </p>

<p>Thus the point of both of them seems to be to allow you to style a polymer element. Why would you use one over the other? The use cases overlap substantially, it seems.</p>

<p>Additional confusion: <code>shared-styles</code> can be imported into <code>custom-style</code>. Why would you do this? Why not?</p>