Hello,
It is possible that the XHTML
output filter is switched off (either for a whole site or particular documents). Please make sure that the
Excluded XHTML filter URLs option in
Site Manager -> Settings -> Your_Site -> Output filter is blank. This way it will work all over the site.
At any rate, be careful with the output filter. It is better to have it switched off due to performance issues. On the other hand, I can clearly imagine scenarios, when it is necessary to use it. Please see a related article:
http://devnet.kentico.com/Blogs/Martin-Hejtmanek/January-2010/Optimization-tip--Write-better-code,-disable-Outpu.aspx.
Best regards
Ondrej Vasil