If I perform a full-text search over xml document with a node containing a html document, and, for example, i'm looking for "head", the engine would return the head html tags, not only the head as body part. Am I wrong?
If full text search is implemented properly, then yes, you are wrong.
Check archives of this (or -dev) mail list, there were discussions on how to implement full text search in the xindice. You can also take a look at Cocoon's XML document indexer implemented on top of Lucene.
Vadim
