As noted by Ashley in discourse, one interesting point of view on HTML Modules is whether they should be considered leaf nodes in the module graph or not. As a leaf-node, only the HTML would be parsed and returned as a document without having any other side-effects (e.g., no running of script, loading of any resources, importing of any other content whatsoever).

This has certain simplifying advantages, but also drawbacks in terms of flexibility of the design.