This issue refers to the security review requested in this issue w3c/vibration#49

Please include in Security Considerations section:

• An explanation on why this spec includes the TTML2 Security Considerations, to be auto-consitent
• Please refer also to Threats/Attacks related to XML
• Draft the Threat Model for the Integrity as with the one we worked togheter in the Issue, considering the Threats considered.

Please add this before it becomes REC, so it's not blocking the various steps to CR.

Thank you