<core-style> should use an element that parses in plain text mode

[tjsavage]

ex.

<style is="core-style" type="polymer/style">
    .span::before { content: "<script>alert(1)</script>"; }
</style>

Using today to do that will alert since it just has random HTML inside it which is both slower for the parser, and will interpret stuff inside it as tags. Instead it should work as above.