Forward-looking: HTTP/3 over QUIC is the replacement for the only thing that hasn't changed on the web in the last thirty years; TCP (transmission control protocol). QUIC uses UDP (user datagram protocol) instead of TCP and is thus simpler and faster.
QUIC was invented by Jim Roskind in 2012. It's been under active development at Google since its announcement in 2013, and over the last couple of years Google has implemented it into Chrome and other services. More than half of Chrome browsers' requests to Google servers are handled by QUIC, and a reasonable portion of Microsoft Edge and Firefox requests are, too.
Fun Fact: The IETF won't accept Google's acronym for QUIC, which is the Quick UDP Internet Protocol.
In 2015, the IETF (the Internet Engineering Task Force, a standards body) took an interest in QUIC. They began developing the standard in parallel to Google in 2016. Their version, IETF QUIC, is considered slightly superior to Google's.
In 2018, the IETF's HTTP and QUIC groups decided that HTTP/3 was going to be defined as HTTP mapping onto QUIC, thus officiating QUIC as a future global standard.
This week, Google announced that Chrome has added IETF QUIC support (draft version h3-29, specifically) and will use HTTP/3 by default on supported websites. Currently, only about 7.5% of websites use HTTP/3 (and QUIC, by extension) which includes all of Google's services and any websites that use Cloudflare and have toggled the setting on.
Technically speaking, Apple beat Google to the punch; Safari 14 arrived with HTTP/3 via IETF QUIC on by default last month. But to Google's credit, they claim that a quarter of Chrome users are already using IETF QUIC.
Google's initial testing has found that IETF QUIC has decreased Search latency by over 2%, and has reduced YouTube rebuffer time by over 9%. In total, client throughput increased by 3% on desktop and 7% on mobile. They think that these numbers will go up as the bugs get ironed out in the coming months.