All you need to parse HTML is regular expressions (to recognize tags) and a stack (to match tags).

Your programming language has a stack -- a call stack.

So in practice all you really need is regular expressions. (Which I tend to call "regular languages" to make a distinction with Perl-style regexes [1], although they work fine too in practice for this case)

Using the call stack in a more functional style is nicer than using the OOP style that s in the Python standard library, which is probably inherited from Java, etc.

I have done this with a bunch of HTML processors for the Oil blog and doc toolchain:

https://github.com/oilshell/oil/tree/master/doctools

It works well in practice and is correct and fast.

Big caveat: this style is only for HTML that I generate myself, e.g. the blog and docs. There are a bunch of rules around matching tags in HTML5 that are subtle. Although one of the points here is that you don't have to do a full DOM-style parse and can ignore those rules for many useful cases.

The other caveat is that HTML has a bunch of rules for what happens when you see a stray < or > that isn't part of a tag. This style makes it a hard syntax error, so it's really a subset of HTML (which has no syntax errors). For my purposes that is a feature rather than a bug, basically following Postel's law.

I meant to write a blog post titled "why/when you can parse HTML with regexes" about this but didn't get around to it.

There is also a caveat where parsing arbitrary name=value pairs with regexes isn't ergonomic, because it's hard to capture a variable number of pairs. However the point is that I wrote 5 or 6 useful and compact HTML processors that don't need that. In practice when you parse HTML you often have a "fixed schema".

Concrete examples are generating a TOC from <h1>, <h2>, etc. and syntax highlighting <pre><code> blocks. Those all work great with the regex + call stack style.

[1] http://www.oilshell.org/blog/2020/07/eggex-theory.html

edit: for completeness, another caveat is that the stack-based style is particularly bad for C++/Rust and arbitrary input because you could blow the stack, although we already limited the problem to "HTML generated ourselves"