The regionAnchorY property of the VTTRegion interface represents the y-coordinate of the region anchor, as a percentage of the region's height.
A number, in the range 0 to 100 inclusive, representing the y-coordinate of the region anchor as a percentage of the region's height. The default is 100.
IndexSizeError DOMExceptionThrown when set to a value that is negative or greater than 100.
In the following example, a new VTTRegion is created, then the value of regionAnchorY is set to 70. The value is then printed to the console.
const region = new VTTRegion();
region.regionAnchorY = 70;
console.log(region.regionAnchorY);