What does the online validator check?

The validator does the following steps:

1. If OME-TIFF extract the OME-XML from the header.
2. Examines the OME-XML block and determines the correct schema.
3. Validates the OME-XML against that schema reporting any errors, and then checks with the other older/newer schemas too but just "for info".
4. Scans the OME-XML and records all object IDs.
5. Scans the OME-XML and checks all Refs and Settings objects point to an ID that is present in the file.
6. If OME-TIFF examined the TiffData nodes and calculates the number of TIFF frames needed to make the file complete, then check the TIFF structure to see if there are enough frames present in the file. If not the file is still valid but incomplete (normally part of a multi-file OME-TIFF). Groups of files cannot be checked together yet.

This all is just checking the structure of the data. No attempt is made to make sure the data values make sense.