embed, source, track, wbr HTML elements not considered empty

[jnns]

BPO	46786
Nosy	@scoder, @jnns
PRs	bpo-46786: Consider embed, source, track, wbr as empty tags #31406

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2022-02-27.14:26:44.874>
created_at = <Date 2022-02-18.10:23:19.542>
labels = ['expert-XML', 'type-feature', '3.11']
title = 'embed, source, track, wbr HTML elements not considered empty'
updated_at = <Date 2022-02-27.14:26:44.874>
user = 'https://github.com/jnns'

bugs.python.org fields:

activity = <Date 2022-02-27.14:26:44.874>
actor = 'scoder'
assignee = 'none'
closed = True
closed_date = <Date 2022-02-27.14:26:44.874>
closer = 'scoder'
components = ['XML']
creation = <Date 2022-02-18.10:23:19.542>
creator = 'jnns'
dependencies = []
files = []
hgrepos = []
issue_num = 46786
keywords = ['patch']
message_count = 3.0
messages = ['413473', '413707', '414157']
nosy_count = 3.0
nosy_names = ['scoder', 'eli.bendersky', 'jnns']
pr_nums = ['31406']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue46786'
versions = ['Python 3.11']

[jnns]

According to the WHATWG, the elements area, base, br, col, embed, hr, img, input, link, meta, param, source, track, wbr are void elements that don't need and therefore shouldn't have a closing tag.

The source view of Firefox 96 shows a warning about an unexpected closing tag 1.

In Python 3.10.2 xml.etree seems to correctly recognize most of them as such and doesn't generate closing tags when using the .tostring() method. A few elements are serialized with a closing tag (<embed></embed> for example).

from xml.etree import ElementTree as etree

void_elements = [
    "area", "base","br", "col", "embed", "hr", "img", 
    "input", "link", "meta", "param", "source", "track", "wbr"
]
for el in void_elements:
    el = etree.Element(el)
    print(etree.tostring(el, method="html", encoding="unicode"))

<area>
<base>
<br>
<col>
<embed></embed>
<hr>
<img>
<input>
<link>
<meta>
<param>
<source></source>
<track></track>
<wbr></wbr>

HTML_EMPTY in Lib/xml/etree/ElementTree.py only contains the following entries:

"area", "base", "basefont", "br", "col", "frame", "hr", "img", "input", "isindex", "link", "meta", "param"

I suppose "embed", "source", "track" and "wbr" should be added to that list.