mutter/tests/data/test-script-timeline-markers.json

{
  "id" : "timeline0",
  "type" : "ClutterTimeline",
  "duration" : 1000,

  "markers" : [
    { "name" : "marker0", "time" : 250 },
    { "name" : "marker1", "time" : 500 },
    { "name" : "marker2", "time" : 750 }
  ]
}