This is rather a consequence of the absolutely arcane way Polymer report the list of elements under a given `<content>`. Every sub-`<content>` and every sub-`<template>` are reported as such - Polymer doesn't do an automatic expansion for you, you have to go inside the `<content>` and `<template>` and fish out the underlying sub-elements yourself - this is what my `SparkWidget.inlineNestedContentNodes` tries to do. But until now I've just ignored `<template>`s, obviously incorrectly. I'll work on this tomorrow.