Reorder steps

Author: MattiasBuelens

index.bs

@@ -9301,10 +9301,10 @@ a reference to the same object that the IDL value represents.
     given a [=buffer source type=] instance |bufferSource|:
 
     1.  If |bufferSource| is [=BufferSource/detached=], then return the empty [=byte sequence=].
-    1.  Let |arrayBuffer| be |bufferSource|.
-    1.  Let |offset| be 0.
     1.  Let |length| be |bufferSource|'s [=BufferSource/byte length=].
     1.  If |length| is 0, then return the empty [=byte sequence=].
+    1.  Let |arrayBuffer| be |bufferSource|.
+    1.  Let |offset| be 0.
     1.  If |bufferSource| is a [=buffer view type=] instance:
         1.  Set |arrayBuffer| to |bufferSource|'s [=underlying buffer=].
         1.  Set |offset| to |bufferSource|'s [=ArrayBufferView/byte offset=].