WebVTT is a W3C standard for displaying timed text in HTML5. Its specification is currently (as of May 2015) in draft stage and therefore not all features are implemented by major players, especially when it comes to positioning. Since there are no recent summaries of WebVTT positioning online and the specification cannot be used by any sane person for quick reference, I'll make a short summary here.
#WebVTT Positioning
Let's start with an example. This file contains the main positioning parameters supported by WebVTT:
WEBVTT
00:00:01.000 --> 00:00:05.000 position:75% align:middle