Fire keypress event for Enter, Shift+Enter, and Ctrl+Enter keys

For web compat and interop with actual behavior in browsers, this change
updates the spec to require firing “keypress” events for the Enter key
and for the combination of the Enter key with the Shift or Ctrl keys.

Otherwise, without this change, the spec is restricting implementations
to requirements for keypress that are different from what browsers need
to implement in order to be compatible with existing web content.

sections/legacy-event-types.txt

@@ -436,10 +436,17 @@ completeness.
 			</table>
 
 			If supported by a <a>user agent</a>, this event MUST be dispatched
-			when a key is pressed down, if and only if that key normally
-			produces a <a>character value</a>. The EVENT{keypress} event type is
-			device dependent and relies on the capabilities of the input devices
-			and how they are mapped in the operating system.
+			when a key is pressed down, if and only if any of the following
+			conditions are true:
+
+			- that key normally produces a <a>character value</a>
+			- that key is the KEYCAP{Enter} key and no modifier was active
+			- that key is the KEYCAP{Enter} key and the KEYCAP{Shift} modifier was active
+			- that key is the KEYCAP{Enter} key and the KEYCAP{Control} modifier was active
+
+			The EVENT{keypress} event type is device dependent and relies on the
+			capabilities of the input devices and how they are mapped in the
+			operating system.
 
 			This event type MUST be generated after the <a>key mapping</a>. It
 			MUST NOT be fired when using an <a>input method editor</a>.