HTML5 video and most web players read captions only as WebVTT, so an SRT from a subtitle site or a video editor has to be converted before it can go into a track element or a video platform's upload. The two formats are close relatives: WebVTT is SRT with a WEBVTT header and a full stop instead of a comma in the timestamps, and it adds positioning and styling that SRT lacks.
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. Top and middle positions and left or right alignment are written as WebVTT cue settings.