Wiki markup for a page that shows html validation failure.
The presence of a TOC in combination with headings in a particular sequence of levels causes w3c html validation to fail. This happens both on a live 1.11.0 wiki and on a freshly installed vanilla copy of mediawiki 1.11.0.
This gives an HTML validation error.
If you remove the Second heading, the page validates OK.
If you remove the subheading instead, the page validates OK.
If you remove the TOC, or replace it with NOTOC in articles with more headings, the page validates OK.
The attached file contains wiki markup of the above example ready for pasting into a new page.