What is an ASS file?
Advanced SubStation Alpha is what you reach for when subtitles have to look like something. The file is divided into sections: script info, a list of named styles each with a font, size, colors, outline, shadow, and margins, and then the events, one line per subtitle referring to a style by name. A single file can put translation at the bottom, a sign translation over the sign itself in a matching font, and karaoke timing that colors each syllable as it is sung.
SSA is the older version of the same idea and is read by the same players; ASS added the styling overrides and effects that made it the standard for fansubbing and for anime releases generally.
What supports it
VLC, mpv, MPC-HC, and the players built into most media centers render ASS properly, as do the tools that burn subtitles into video. Browsers do not support it at all, most streaming uploads reject it, and TV boxes vary. Fonts are another catch: a file styled with a font the viewer does not have falls back to something else and the careful layout goes with it.
Converting ASS to SRT
When something will not take the file, convert it: the text and the timings come across, and everything that made it ASS does not. Positioned signs collapse to ordinary bottom-of-screen lines, karaoke timing becomes a plain line of text, and styles disappear.
If the styling matters and the target will not read ASS, the other route is to burn the subtitles into the video itself, which turns them into pixels that cannot be turned off.
Frequently asked questions
What is the difference between ASS and SSA?
ASS is the newer revision, with styling overrides, more effects, and better positioning. Players that read one read the other, and files usually carry the .ass extension either way.
Why do my subtitles look wrong on another device?
The fonts named in the styles are not installed there, so the player substitutes something else. Either stick to common fonts or burn the subtitles into the video.
Can I use an ASS file on a website?
Not directly. Browsers only read WebVTT. Convert to VTT for the words and timings, or burn the styled subtitles into the video.
Does converting to SRT lose anything?
Everything except the words and the timings: styles, positions, karaoke effects, and per-line overrides all go.