SSF

From Aegisub Wiki

SSF (Structured Subtitles Format), much like USF is an XML based subtitle format developed by Gabest. It also borrows ideas from Cascading Style Sheets. It's expected to be fully supported by the next release of MPC and VSFilter. However, there are no programs that support editing in that format.

The format has received a lot of criticism for its extreme verbosity in order to accomplish simple tasks, which makes it unpractical to write by hand. Aegisub has no support for this format, and it most likely won't unless high interest is shown in it.