what is xml 'is' attribute

While using the vaadin polymer showcase to seed my prototype, I've come across an unfamiliar uibinder construct: is='xxx'

Is there documentation anywhere on this attribute?  Searching for "is" attribute is impossible.

Some excerpts showing the attribute from

http://vaadin.github.io/gwt-polymer-elements/demo/#iron/IronInputSample

<style is="custom-style">

input[is=iron-input] {}

</style>

<template ui:field="bindTemplate" is="dom-bind">

<input is="iron-input" />

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