simple_video_progress.js

// "videoTag" is <video> element pulled from the DOM  
var videoPercentageRate = this.duration / 100;
var videoCompletionRateIntervals = [25,50,75,100];

videoTag.play = function() {
    var percentageCompleted = (videoTag.currentTime / videoTag.duration) * 100
    console.log(percentageCompleted);     

    if (percentageCompleted >= videoCompletionRateIntervals[0]) {
        console.log(videoCompletionRateIntervals[0], '% of video completed');
        // GA Event
        videoCompletionRateIntervals.splice(0,1); 
    }

    if (videoCompletionRateIntervals.length == 0) {
        videoTag.play = null;
    }
}