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 a Sequence Ontology OBO file).
This GFF3 validator is part of the GenomeTools distribution which you can download to your computer. Use the gt 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).