Define "usual constructor steps"

This is for use in [HTMLConstructor] in whatwg/html#4915.

Author: domenic

index.bs

@@ -11126,13 +11126,13 @@ and is described in more detail in [[#interface-prototype-object]].
 Note: Since an [=interface object=] is a [=function object=]
 the <code>typeof</code> operator will return "function" when applied to an interface object.
 
-<div algorithm>
+<div algorithm="create an interface object">
 
     The [=interface object=] for a given [=interface=] |I|
     with [=identifier=] |id| and in [=Realm=] |realm|
     is <dfn lt="create an interface object">created</dfn> as follows:
 
-    1.  Let |steps| be the following steps:
+    1.  Let |steps| be the following steps, known as the <dfn export>usual constructor steps</dfn>:
         1.  If |I| was not declared with a [=constructor operation=],
             then [=ECMAScript/throw=] a {{ECMAScript/TypeError}}.
         1.  If {{NewTarget}} is <emu-val>undefined</emu-val>, then
@@ -11186,6 +11186,9 @@ the <code>typeof</code> operator will return "function" when applied to an inter
     1.  [=Define the static operations=] of [=interface=] |I| on |F| given |realm|.
     1.  Return |F|.
 
+    Other specifications may require that, under certain circumstances, a different set of steps
+    replace the [=usual constructor steps=].
+
 </div>