If you need to create new styled subtitles today, start with (same syntax, more features) and save as .ass . But if you receive an .ssa file, you can work with it directly in most modern tools.
(convert SSA to SRT):
[Script Info] Title: My Subtitles ScriptType: v4.00 PlayResX: 640 PlayResY: 480 [V4 Styles] Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, AlphaLevel, Encoding Style: Default,Arial,20,&Hffffff,&Hffff00,&H000000,&H808080,0,0,1,1,1,2,10,10,10,0,0