Exciting with Chrome 91 is WebTransport being added as an experimental origin trial. WebTransport is a new protocol framework / standard for communicating with a remote server using a secure multiplexed transport method. WebTransport is an alternative to WebSockets and provides an API for bi-directional traffic using UDP-like datagrams.
WebTransport is currently available as an Editor's Draft via the W3C on GitHub . WebTransport there is summed up as " to send data to and receive data from servers. It can be used like WebSockets but with support for multiple streams, unidirectional streams, out-of-order delivery, and reliable as well as unreliable transport. "
Meanwhile being promoted out of the original trial phase is the WebAssembly SIMD support. WebAssembly SIMD is for making use of the system's SIMD instruction capabilities within WebAssembly but in a platform independent manner.
Details on these and other changes can be found via the Chromium.org blog . ChromeStatus.com also has a list of Chrome 91 changes building up including the GravitySensor API, WebSockets over HTTP/2, and the other interesting bits.