I thought I understood what the Shadow DOM was. Then I read w3.org/TR/shadow-dom

Any element in a shadow tree can be a shadow host, thus producing nested shadow trees. A shadow tree is nested when its shadow host is itself a part of a shadow tree. Conversely, a shadow tree A is said to be nesting the shadow tree B if B is nested by A. If a shadow host is declared in the document, the document is the nesting tree of its shadow trees.

wat

One case that deserves special consideration is the situation when an insertion point is a child node of another shadow host. In such situations, the nodes distributed into that insertion point must appear as if they were child nodes of the shadow host in the context of distribution within the shadow tree, hosted by said shadow host. Thus, the nodes distributed to a shadow tree could have already been distributed by the nesting tree. The effect of a node being distributed into more than one insertion point is called reprojection.

waaaaat

While pondering if his company should start looking for a new designer

wa

Well done, Bob! With the cup of coffee still half-full, the work is complete. Recognizing his awesomeness, Bob returns to teaching n00bs the ways of WoW.

seriously

// TODO(alice): Check designer's desk for hallucinogens.

?

takes the cake for using shadow tree composition in such a cool way.

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davewiner I disagree that browser makers should “make sure they don’t break bootstrap apps”. They should concentrate on following specs, and markup authors should do the same.