Buggy wikitext markup of course .. but, Parsoid should not be generating fosterable output, if it can help it. Caused by this wikitext line:

{{Election box begin |title=[[United Kingdom general election, 2010|General Election 2010]]: Croydon South}} <ref>[http://ukpollingreport.co.uk/guide/seat-profiles/croydonsouth Croydon South] UK Polling Report</ref>

The <ref> should be *inside* the template rather than outside it. This causes Parsoid to generate

<tr> <span about="#mwt3" class="reference" id="cite_ref-1" rel="dc:references" typeof="mw:Extension/ref" data-mw='{"name":"ref","body":{"id":"mw-reference-text-cite_note-1"},"attrs":{}}'><a href="#cite_note-1">[1]</a></span>
</tr>

That span of course gets fostered about and breaks the about-grouping by inserting itself before the table! And, causes erroneous serialized output. That error can be reproduced with this wikitext snippet:

{{echo|{{{!}}
{{!}}-}}<ref>foo</ref>
|}

I will edit the page and fix the problem for now, but leaving this bug report behind to see what can be fixed.

Change 204439 had a related patch set uploaded (by Subramanya Sastry):
T96197: <ref> marker metas should remain fosterable in HTML builder

https://gerrit.wikimedia.org/r/204439

Change 204439 merged by jenkins-bot:
T96197: <ref> marker metas should remain fosterable in HTML builder

https://gerrit.wikimedia.org/r/204439