The tree composition algorithm states: <http://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#dfn-tree-composition>

"The insertion points or shadow insertion points in fallback content must not participate in composition."

This should be clarified by formalizing it in the algorithm or clarifying the meaning of “insertion point” and “shadow insertion point”.

For example, is the distribution algorithm when referred to by the tree composition algorithm in step 4.2 thus part of composition, or separate to it? If it is separate, then

<content select="a">
  <content select="b"></content>
</content>

is a way to say “distribute a and b here; if there are a, output them, otherwise output b.”

If the distribution algorithm is a sub-part of composition then per the prose regulation the meaning is roughly "distribute a here; if there are a, output them.”

This should be clarified by formalizing the prose regulation in the algorithm. For example, if the intended meaning is the latter, then the term “insertion point” could be tweaked to include ”…that does not have any insertion point as an ancestor.”