The second prerelease of version 2 of the e-invoicing library Mustang can convert invoices into HTML. Furter details can be obtained from the release notes.
Since August 15th a prerelease of version 2 of the open-source e-invoicing library Mustang contains the validator ZUV, which now also validates XRechnung and supports ZF2.1.1. Further details are in the Release Notes.
The releases of the open source e-invoice library Mustang from February 6th, May 26th and June 14th 2020 allowed among others the specification of a shipping address, invoices for intra community supply and corrected gross prices in ZUGFeRD 2 invoices.