2.2.2 [W3C-TTML-1.0] Section 8, Styling
The specification states:
Styling attributes are included in TTML to enable authorial intent of presentation to be included within a self-contained document. This section describes the semantics of style presentation in terms of a standard processing model. TTML Processors are not required to present TTML documents in any particular way; but an implementation of this model by a TTML Presentation Processor that provides externally observable results that are consistent with this model is likely to lead to a user experience that closely resembles the experience intended by the documents' authors.
IE10 Mode, IE11 Mode, and EdgeHTML Mode (All Versions)
Styling is not supported.