OBJECT tag is self-nestable

[coffee2code]

As #6870 advocated for the q tag, likewise the object tag is self-nestable.

As proof of its validity, the W3C's HTML4 documentation for object includes an example of the self-nesting of that tag . Down a bit from ​this link , their example is:

<!-- First, try the Python applet -->
<OBJECT title="The Earth as seen from space" 
        classid="http://www.observer.mars/TheEarth.py">
                    <!-- Else, try the MPEG video -->
  <OBJECT data="TheEarth.mpeg" type="application/mpeg">
                    <!-- Else, try the GIF image -->
    <OBJECT data="TheEarth.gif" type="image/gif">
                    <!-- Else render the text -->
     The <STRONG>Earth</STRONG> as seen from space.
    </OBJECT>
  </OBJECT>
</OBJECT>

And ​another example they used for the param tag (scroll back a few lines from that link target to see the example):

<OBJECT id="obj1">
   <PARAM name="param1">
   <OBJECT id="obj2">
      <PARAM name="param2">
   </OBJECT>
</OBJECT>

A unit test patch will be forthcoming.

Aforementioned path.

Unit test included as part of ​[UT661].

Might as well include this much more minor patch in 3.5 if #1597 looks like it'll go in.

In [21250]:

HTML object tags are self-nestable. props coffee2code. fixes #20401.