VTTCue.lineAlign and VTTCue.positionAlign are not supported. In the WebVTT syntax these are specified after a "," in position and line settings. Spec: http://w3c.github.io/webvtt/#webvtt-cue-position-alignment http://w3c.github.io/webvtt/#webvtt-cue-line-alignment http://w3c.github.io/webvtt/#parse-the-webvtt-cue-settings http://w3c.github.io/webvtt/#the-vttcue-interface Tests: http://w3c-test.org/webvtt/api/VTTCue/lineAlign.html http://w3c-test.org/webvtt/api/VTTCue/positionAlign.html These are supported in Gecko. Bug for Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=633690 I think the use case is basically better control in positioning things. See http://w3c.github.io/webvtt/#example-0c2976c9 for an example. 00:00:00.000 --> 00:00:04.000 position:10%,line-left align:left size:35% Where did he go? Without position-align you need to set position to some value that doesn't correspond to what the author actually intends: 00:00:00.000 --> 00:00:04.000 position:27.5% align:left size:35% Where did he go? 27.5% here is: intended position (10%) + size/2 (35%/2). Not intuitive. Having this also makes the mapping from other subtitle formats to WebVTT (and back) more straightforward.
lineAlign & positionAlign.html passing in STP180: https://wpt.fyi/results/webvtt/api/VTTCue/lineAlign.html?label=experimental&label=master&aligned https://wpt.fyi/results/webvtt/api/VTTCue/positionAlign.html?label=experimental&label=master&aligned ___ Marking this as 'RESOLVED CONFIGURATION CHANGED'. Thanks! Please reopen, if there is something else to fix here.