The first fragment of IDL defined in 4.10.20 is invalid as a standalone IDL fragment. In particular, it cannot be extracted and inserted into an IDL file without producing a syntax error. All other IDL fragments in the spec do not have this problem. This presents a problem for tools that harvest IDL from the HTML spec in order to construct a valid IDL file corresponding to the IDL entities defined in the spec. Further, at present, the current definition is semantically incomplete from the IDL perspective since it fails to bind the two affected interfaces with the members defined by this fragment.

In order to correct, I would suggest wrapping these methods in a new [NoInterfaceObject] interface definition which is then used in "implements" statements for the HTML{Input,TextArea}Element interfaces. 

For example, add the wrapper:

interface TextSelection {
  ...
};
HTMLInputElement implements TextSelection;
HTMLTextAreaElement implements TextSelection;

where ... is the current incomplete fragment:

  void select();
           attribute unsigned long selectionStart;
           attribute unsigned long selectionEnd;
           attribute DOMString selectionDirection;
  void setRangeText(DOMString replacement);
  void setRangeText(DOMString replacement, unsigned long start, unsigned long end, optional SelectionMode selectionMode = "preserve");
  void setSelectionRange(unsigned long start, unsigned long end, optional DOMString direction = "preserve");

Note that this problem also holds with HTML5.1 nightly.