I have a few xml files and they don't get validated when it is loaded. However, there is a validator online I just found
http://tools.decisionsoft.com/schemaValidate/ that will validate your file against your schema. I ran my files through it and guess what... my schemas were invalid. Then I "make" them valid.
I'm sure if you use DTD then when the xml file is loaded in the browser, it will prompt error if your DTD doesn't match.