Especially in the introduction phase of a new standard, it is important to make sure that the APIs of different software systems use the standards in the same way and conform to the rules of the specifications.

Once a workflow is established, the exchanged documents need to conform to the established APIs.

Take for example the following real-world use case scenario: a freelancer creates subtitles for a media service company which in turn got a contract from a streaming company to deliver subtitles for a complete season of a video series.

The media service company and the streaming company agree on IMSC as the technical baseline for subtitle XML files, but they may also apply their own set of “house style rules”. So at the gateways of the service company and the streaming provider, overlapping but also separate checks are executed.

The Institute für Rundfunktechnik (IRT) ^[8]was well aware of this and other scenarios such as the “in-house” exchange of subtitle files and aimed for a better support for them.

In 2013, the IRT had already been active in TTML standardization, especially in the definition of the EBU-TT profiles.^[9] But although W3C XML Schema (XSD) files existed, non-conformant documents were still produced and distributed.

This was due to a lack of knowledge about the existence of the XSD itself, its application or about the meaning of the implied rules and resulted in the new goal to build a more user-friendly validation alternative. The main inspiration came from the W3C HTML validation service.

In the process of the installation of their own service, the IRT went through different stages:

In the following, we describe the implementation of the subcheck service by BaseX GmbH and the IRT.