I want a fully implemented and extensible SSML parser for Web Speech

Several web services (requiring some form of payment and/or end-user agreement) offer SSML parsing, including Amazon Alexa, IBM Watson Bluemix, and Google Actions. SSML parsing should be open source and free, without requiring payment and/or EUA/licensing.

I am currently using my own code, which parses several—though not all—of the SSML elements and attributes described in the SSML spec. The implementation, when completed, should be extensible, capable of being adjusted for future development of speech synthesis technologies.