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From: https://lists.w3.org/Archives/Public/public-webapps/2015AprJun/0052.html "Mostly for performance it would be nice if the composed tree could be serialized and cached. That way first render only requires HTML and CSS with JavaScript kicking in at the end. We reasoned it might not be too hard to add something like <shadowroot> given our experience with <template>. The only difference would be that <shadowroot> itself would also not be appended to the tree and that the DocumentFragment nee ShadowRoot is associated with "its parent"."
Here's a fairly simple implementation of this using custom elements and templates: http://jsbin.com/xuheb/4/edit I think this is a great example of: a) using type extensions to avoid more hard-coded parsing behaviors b) extensible web in action -- no need to wait on spec/browsers to ship this c) custom element upgrades being helpful in avoiding additional tree traversals and deferring running of the script until whenever you actually need the tree.
Moved to https://github.com/w3c/webcomponents/issues/71