Ref: whatwg/html@172cccf

The "selectedcontent" element was added.

"Contexts in which this element can be used:" and/or "Content model:" were changed.

• button element
• div element
• hr element
• legend element
• noscript element
• optgroup element
• option element
• select element

In the optgroup element;

• The label attribute must be specified.
  　　↓changed
• The label attribute must be specified if the optgroup has no child legend element.