libxslt is now unmaintained 
in my “why are y’all rewriting everything in Rust while critical infrastructure goes unmaintained” era
@Lady It actually has a surprisingly small set of rdepends within Ubuntu. Some important packages, like Postgres, but mostly desktop software besides Postgres, Icecast, and Asterisk, and it's hard to say the latter have all too many users. There's Ruby's Nokogiri, but given the near-weekly security updates to that, I might almost prefer someone Rewrite it in Rust (or literally anything).
@aschmitz (i think actually webkit and chrome depends on libxml2/libxslt and gecko has their own c++ implementation.)
A. Wilcox delicately
@Lady WebKit uses libxslt, at least it did when I packaged it.
Firefox has its own custom HTML 5 parser, originally developed around 2009 and somewhat based off the HTML 5 standard parsing algorithm and a Java reference implementation, unimaginatively just called "parser". It does vendor a copy of Expat for pure XML.
See also:
https://johnresig.com/blog/html-5-parsing/
https://hacks.mozilla.org/2010/05/firefox-4-the-html5-parser-inline-svg-speed-and-more/
https://hg-edge.mozilla.org/mozilla-central/file/tip/parser
@awilfox @Lady WebKit developer here! Libxml and libxslt are used for... Well, XML and XSLT support (mainly the latter). The main HTML/XHTML parser is custom and heavily tested (and fuzzed) like the rest of WebKit. For those truly concerned about bugs in those libraries, both @WebKitGTK and @WPEWebKit may be built without them but losing a bit of (somewhat niche) functionality.
@awilfox @Lady @WebKitGTK @WPEWebKit the relevant code is in https://github.com/WebKit/WebKit/tree/main/Source/WebCore/html/parser
As well as xmlto, notably used by DocBook (which is a pile of XSLT).
Used https://packages.gentoo.org/packages/dev-libs/libxslt/reverse-dependencies and https://packages.gentoo.org/packages/app-text/xmlto/reverse-dependencies as sources.
https://gitlab.gnome.org/GNOME/libxslt/-/blob/master/libxslt.doap?ref_type=heads
EDIT: Ah, hasn't been active on the gitlab except to leave libxslt… https://gitlab.gnome.org/veillard
@lanodan @Lady Kind of, yes. Though systemd and ninja only need it for building, nginx only(?) needs it for libnginx-mod-http-xslt-filter, and php uses it for php-xml. And we're both leaving out things, but I was just sort of surprised by how few things actually seem to have firm dependencies on it. On the other hand, there's enough that if it stopped working with, say, a new glibc/musl version, there would probably be a fix in short order.
Distros can help a bit on security issues but packagers jumping in codebases they aren't much familiar with doesn't always works so well.
@aschmitz and yes i would love for someone to rewrite it as well (and make it support XML1.1, and XSLT/XPATH 2.0, and 3.0…), but nobody IS going to do that, because it’s very hard, and in the meantime libxml2 is what is available on every platform, and it still has bugs i would love to see get solved