H in DIV Causes HTML Explosion in XF
Under XenForo, using an H tag inside a DIV tag causes an HTML explosion. Content after the H tag is not included in the final output of the page. Apparently this was a separate problem (H tags prematurely ending the content), but makes more catastrophic results inside DIV tags, and this only occurs in XenForo installations.
Fixed for the next release. A child property of the parser contained a backreference to the parser. While the parser could have multiple instances, the property used a singleton of itself, and so the backreference resulted in each new parser instance to overwrite the previous ones. Quite messy indeed.
The question remains whether this is a more common problem throughout the parser with other instantiable properties, which may not be isolated only to XenForo installations. I must delve deeper...