After building an EPUB with bber build epub, b-ber will run the compiled EPUB through the EPUBcheck validator.

EPUBcheck is an open source tool developed by the International Digital Publishing Forum (IDPF) that is used to ensure the quality of EPUB files. Third party vendors will almost always require that any EPUB files submitted to them pass EPUBcheck without errors.

Detailed information about how EPUBcheck works can be found here.

Every error message produced by EPUBCheck is preceded by the resource name and the line number where the error occurred.

See more about errors here.