ASS styling is not understood by simple players, and it is lost when converting to plain formats. VTT is what web video players and browsers display natively. ASS is at home in anime and karaoke subtitles with fonts, colours and positioning; VTT is the usual choice for captions on web pages and in HTML video players.
The text is read as UTF-8 or UTF-16, whichever the file is. An older file in another code page is read using the charset it declares, or as Windows-1252 if it declares none, so accented letters in an unlabelled legacy file can come out wrong. A file that is not text, such as the picture-based subtitles of a DVD, is refused instead of being converted to nonsense. Of the styling, only italic, bold, underline and the position on screen (top, middle or bottom, left, centre or right) travel between formats; fonts, colours, outlines and effects do not. Each line takes its italic, bold, underline and alignment from its style and from override tags; drawing commands and karaoke effects are ignored. Top and middle positions and left or right alignment are written as WebVTT cue settings.