@taleinat I made minor changes to comment texts and will consider a few more.

I think I found a mismatch between indent() and one of its calls.

    def indent(self, amt=1):
        "Change indent (+1, 0, -1) and tags."
        self.level += amt
        self.tags = '' if self.level == 0 else 'l'+str(self.level)
    ...    
        if tag in ['h1', 'h2', 'h3']:
            self.indent(0)  # Clear tag, reset indent.

The comment asserts that self.indent(0) will set self.level to 0 and then self.tags to ''. But it leaves self.level alone and only resets self.tags if self.level was already 0. As it turns out, the latter is always true, as I would expect it to be if +1s and -1s are properly matched. So I think the call should be replaced with self.tags = '', possible preceded by assert self.level == 0.

I think I found a mismatch between indent() and one of its calls.

I noticed that too but didn't investigate.

I suggest separating into three methods: indent(), dedent() and reset_indent().

After more thought, I went with assert at the beginning of the block, as I am pretty sure level != 0 would indicate an error somewhere. The set tags at the end of the block, as done for other blocks.

Thanks @terryjreedy for the PR 🌮🎉.. I'm working now to backport this PR to: 3.7, 3.8.
🐍🍒⛏🤖

GH-15683 is a backport of this pull request to the 3.8 branch.

GH-15684 is a backport of this pull request to the 3.7 branch.