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HTML tends to compress well, typically consisting of text interspersed with repetitive tags. Transparently compressing it for download is a fairly obvious optimization to save bandwidth.
When did Web servers start routinely doing this? Mozilla's development pages mentions that HTTP/2 in 2015 started compressing headers, but doesn't seem to say anything about compression of text in document bodies.
Compression of document bodies is negotiated between clients and servers (and proxies), using notably the Accept-Encoding header: the client indicates which compression algorithms it supports, and the server chooses one as appropriate for the content being served (e.g. a JPEG file shouldn’t be compressed again) and the context of the negotiation (e.g. TLS sessions can be compressed, so there’s no point in serving compressed content in that context).
HTTP/1.0 (1996) already supported compress and gzip; more were added in HTTP/1.1. Compression was supported before HTTP/1.0 was published, at least by CERN httpd (in 1994). The use of compression was common early on: GIF and JPEG for images of course, and dynamically-negotiated compression for text. I don’t have historical references for the latter but I do remember it being a common part of setting up a web server in the nineties (although mis-configured servers weren’t rare).
http/0.9 didn't have any headers, and so couldn't support compression.
On the other hand, http/1.0 (rfc1945, May 1996) already had that support. It was already on draft-00 (as Content-Encoding: x-gzip), dated November 28, 1994.
Do note that HTTP/1.0 was first implemented, then published as an informational RFC. See one of the first messages on the working group.
This 1992 version of HTTP already included Content-Encoding (despite the title, I suspect the page contents might reflect a later year than 1992, though)
First version of gzip program (gzip 0.1) was first publicly released on 31 October 1992, and version 1.0 followed in February 1993. (Wikipedia)
Thus, compression support must have been added to http around 1992-1993.
Of note, NCSA Mosaic (started around those dates) doesn't support Content-Encoding header.
Since Content-Encoding is a negotiated header in HTTP, it is transparent, as soon as clients started using HTTP/1.0 the servers would have started compressing responses when they could (i.e. almost on every static file they served).
I surfed the entire web once -- I fired up NCSA Mosaic, found there were only about a dozen web sites up at the time, and pulled up at least the front page on all of them. I don't recall Netscape 0.9 (came out in 1994) supporting HTTP compression. But my recollection is by 1995 or so, HTTP compression was already fairly widespread.
