https://developer.mozilla.org/en-US/docs/Web/API/Document/querySelector

The Document method querySelector() returns the first Element within the document that matches the specified selector, or group of selectors. If no matches are found, null is returned.

https://www.w3schools.com/cssref/css_selectors.asp

CSS selector works in the same way in JavaScript:

https://www.w3schools.com/jsref/met_audio_play.asp

var vid = document.getElementById("myVideo");
vid.currentTime = 5;

The currentTime property sets or returns the current position (in seconds) of the audio/video playback.

https://developer.mozilla.org/en-US/docs/Web/API/Element/classList

The Element.classList is a read-only property that returns a live DOMTokenList collection of the class attributes of the element. This can then be used to manipulate the class list.

Using classList is a convenient alternative to accessing an element's list of classes as a space-delimited string via element.className.

https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/transitionend_event

The transitionen event is fired when a CSS transition has completed. In the case where a transition is removed before completion, such as if the transition-property is removed or display is set to none, then the event will not be generated.