2.1.61 [W3C-HTML52] Section 4.11.2. The summary element

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V0400: The summary element is not supported

The specification states:

 4.11.2 The summary element
     ...
     The first summary child element of a details element represents a summary, caption, 
     or legend for the rest of the contents of the parent details element, if any.

EdgeHTML Mode

The summary element is not supported.