What is SRT?
SubRip subtitles: a number, a timestamp, a line of text, a blank line, repeat. That's the entire format. It carries no styling and no positioning, which is precisely why every player, TV and video tool on earth reads it.
What is WebVTT?
The subtitle format built for HTML5 video - what a browser loads into a track element. It's SRT with a header, a full stop instead of a comma in the timestamps, and optional positioning and styling that most people never touch.