Email templates for MS Outlook require conditional CSS. A line of code for that looks like this: <!--[if (gte mso 9)|(IE)]>
That is an if block that will be called if the version of MSOutlook receiving the email is greater than 9 or if it’s any version of Internet Explorer.

Emails sent to MSO users are displaying the code for the conditional CSS:

If you look closely you’ll see that the line of code that I used as an example looks like this: <!--[if (gte mso 9)|(IE)]&gt;
The transformation of > into it's HTML counterpart, > is not allowing the if block to adequately close.

The source of that transformation comes from: twisted.web._flatten.escapedComment where this line of code exists: data = data.replace(b"--", b"- - ").replace(b">", b">")

That was introduced in this PR

In the docstring of test_commentEscaping this statement was made: "Comments start with and never contain -- or >." That is not correct given that we close if statements with a >
i.e. <!--[if (gte mso 9)|(IE)]>

Also, the referenced XML spec defines comment as:

Char	   ::=   	#x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF]	/* any Unicode character, excluding the surrogate blocks, FFFE, and FFFF. */
Comment	   ::=   	'<!--' ((Char - '-') | ('-' (Char - '-')))* '-->'

And > would be #x3E.

To solve this issue we patched twisted.web._flatten.escapeComment at runtime, removing .replace(b">", b">")

I am creating this issue to introduce the problem and subsequently I will submit a PR that aims to solve the problem.

Testing environment

• Python version 3.10.4
• Twisted version 22.10.0
• ProductName: macOS
  ProductVersion: 13.1
  BuildVersion: 22C65

$ pip freeze
alabaster==0.7.13
appdirs==1.4.4
astor==0.8.1
astroid==2.14.1
attrs==22.2.0
Automat==22.10.0
Babel==2.11.0
CacheControl==0.12.11
cachetools==5.3.0
certifi==2022.12.7
cfgv==3.3.1
chardet==5.1.0
charset-normalizer==3.0.1
click==8.1.3
click-default-group==1.2.2
colorama==0.4.6
ConfigArgParse==1.5.3
constantly==15.1.0
coverage==6.6.0b1
dill==0.3.6
distlib==0.3.6
docutils==0.19
exceptiongroup==1.1.0
filelock==3.9.0
hyperlink==21.0.0
hypothesis==6.65.2
identify==2.5.17
idna==3.4
imagesize==1.4.1
incremental==22.10.0
isort==5.12.0
Jinja2==3.1.2
lazy-object-proxy==1.9.0
lockfile==0.12.2
lunr==0.6.2
MarkupSafe==2.1.2
mccabe==0.7.0
msgpack==1.0.4
nodeenv==1.7.0
packaging==23.0
platformdirs==2.6.2
pluggy==1.0.0
pre-commit==3.0.2
pydoctor==22.9.1
pyflakes==2.5.0
Pygments==2.14.0
PyHamcrest==2.0.4
pylint==2.16.0
pyproject_api==1.5.0
pytz==2022.7.1
PyYAML==6.0
readthedocs-sphinx-ext==2.2.0
requests==2.28.2
six==1.16.0
snowballstemmer==2.2.0
sortedcontainers==2.4.0
Sphinx==5.3.0
sphinx-rtd-theme==0.5.1
sphinxcontrib-applehelp==1.0.4
sphinxcontrib-devhelp==1.0.2
sphinxcontrib-htmlhelp==2.0.1
sphinxcontrib-jsmath==1.0.1
sphinxcontrib-qthelp==1.0.3
sphinxcontrib-serializinghtml==1.1.5
toml==0.10.2
tomli==2.0.1
tomlkit==0.11.6
towncrier==22.12.0
tox==4.4.4
-e git+https://github.com/twisted/twisted.git@59783c7c2e40868283395b2a88125618b254631c#egg=Twisted
twistedchecker==0.7.4
typing_extensions==4.4.0
urllib3==1.26.14
virtualenv==20.17.1
wrapt==1.14.1
zope.interface==5.5.2