Wikimedia Bugzilla is closed!

MediaWiki adds an ID attribute to each heading on a page using the heading's contents as name. The HTML specification requires this name to be 
unique in a document.

It is possible to use headings with the same name than existing MediaWiki IDs in the page (like "toc", "column-one", or "siteNotice") resulting in 
having the same ID twice and thus breaking the HTML.

The usefulness of these IDs might be limited anyway since the contents of wiki pages and the names of these IDs might change sudden and often 
and since the A tag where these IDs are put in is closed directly afterwards so there's no content that could be addressed by a stylesheet using these 
IDs.