GFF3 online validator
Use this form to upload a GFF3 annotation file (up to 50 MB, can be .gz or .bz2 compressed) which is then validated against the GFF3 specification (potentially also using the current Sequence Ontology OBO file).
This GFF3 validator is part of the GenomeTools distribution which you can download to your computer. Use the gff3validator tool to validate your own – possibly larger – GFF3 files (with option -typecheck so to use the Sequence Ontology). Use the gff3 tool with option -tidy to tidy them up (-help shows further options).