What browser APIs are important for reproducing HLS/DASH in the browser?

The following browser APIs are important for playing HLS/DASH in a browser:

• Media Source Extensions (MSE) — allows dynamic creation of media streams by loading video and audio segments, which is necessary for adaptive streaming.
• Encrypted Media Extensions (EME) — provides support for DRM if the stream is protected.
• Fetch API / XMLHttpRequest — for loading segments and playlist manifests.
• Web Audio API — sometimes used for audio processing.

Example of using MSE for playing HLS/DASH:

const video = document.querySelector('video');
const mediaSource = new MediaSource();
video.src = URL.createObjectURL(mediaSource);
mediaSource.addEventListener('sourceopen', () => {
  const sourceBuffer = mediaSource.addSourceBuffer('video/mp4; codecs="avc1.64001e, mp4a.40.2"');
  fetch('segment1.mp4')
    .then(response => response.arrayBuffer())
    .then(data => sourceBuffer.appendBuffer(data));
});

Thus, MSE is a key API for implementing custom players with support for adaptive streaming.