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If I try to put something like this into a question or answer

<!-- <br>s are important here -->

the first > after the "br" is matched to the first < before the "!--", resulting in this being displayed:

s are important here -->

This happens in both previews and actual posts. I'd say it's a bug. HTML comments shouldn't be treated as regular HTML tags; <!-- should only be matched to -->.

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  • 6
    So what you're saying is that > is ending your HTML comment even though only --> should end your HTML comment, as explained here? In other words, <!-- this is a comment --> and <!-- this is a comment > are being rendered identically (hidden) even though the latter isn't a valid HTML comment?
    – Chris Frederick
    Commented Jan 26, 2012 at 20:34
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    @Chris: hmm, I think yes, exactly. Look here, you'll see that html isn't broken by > there
    – Martin.
    Commented Jan 26, 2012 at 20:36
  • @Chris: no, actually I need to use >--> in order to break it (to terminate the comment). Look at my last edit
    – Martin.
    Commented Jan 26, 2012 at 20:38
  • @Martin Could you edit that back into your question? It sounds like a lot of folks are getting confused about the difference between HTML comments, Stack Overflow comments, and HTML comments in Stack Overflow comments. You're talking about HTML comments in Stack Overflow questions and answers.
    – Chris Frederick
    Commented Jan 26, 2012 at 20:39
  • @Chris: Feel free to edit it back, because I'm not sure what should I edit back
    – Martin.
    Commented Jan 26, 2012 at 20:40
  • @MatthewRead: Glad you got it now :)
    – Martin.
    Commented Jan 26, 2012 at 20:46
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    @Martin: This is not a bug. Please do not add that tag in.
    – animuson StaffMod
    Commented Jan 26, 2012 at 20:48
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    @animuson: yes, that's a bug. HTML comment is being malformed by adding >
    – Martin.
    Commented Jan 26, 2012 at 20:50
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    @AdamRackis: It doesn't matter. I didn't say that team should fix it. It is a bug, so I reported it.
    – Martin.
    Commented Jan 26, 2012 at 20:54
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    Stop adding the bug tag, this isn't one. HTML comments are not among the allowed HTML tags and get removed. Conveniently, this looks like they were treated as comments, but they're not.
    – balpha StaffMod
    Commented Jan 26, 2012 at 21:01
  • @balpha: I didn't add one for 14 minutes, because I wanted to wait for you
    – Martin.
    Commented Jan 26, 2012 at 21:02
  • Can we have a better markdown-ish way to specify language formatting then? <!-- language: lang-foo --> is such a pain to type. How about [lang:foo]?
    – sarnold
    Commented Jan 26, 2012 at 22:49

3 Answers 3

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17

It looks like this is :

It should be mentioned that while a > is a valid character in a comment from both the W3C specification and the above Regex, it's not going to work correctly in our Markdown, because the tag sanitizer will eat it:

<!-- love > hate -->

turns into

hate -->

Personally I would not expect the tag sanitizer to take precedence over this, but it's not my call.

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13

The short answer, according to balpha ♦, appears to be that Stack Exchange does not support HTML comments in Markdown: it only supports a very strict subset of HTML. You may be able to put HTML comments in your posts, and they may be hidden in the rendered output, but they're not really guaranteed to work.

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    So the fact that HTML comments produce invisible content in an SO post is merely a by-product of the fact that it's being parsed as an (invalid) HTML tag, and not an HTML comment?
    – user159834
    Commented Jan 26, 2012 at 21:12
  • @Madmartigan That's the way I interpret it.
    – Chris Frederick
    Commented Jan 26, 2012 at 21:13
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    So then <!-- this --> gets interpreted as a "!-- this --" HTML tag, and it's invalid so it's just not displayed? Just like if you used <hello this is a "comment"> to hide text? Looks like the fact that HTML comments work in the first place is the only "bug" then...
    – user159834
    Commented Jan 26, 2012 at 21:14
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    @Mad Except for a very few occasions where they're (ab)used as metadata containers (e.g. specifying the language for the syntax highlighter), yes, it's a byproduct of being considered invalid.
    – balpha StaffMod
    Commented Jan 26, 2012 at 21:16
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    I've always used this <!-- --> to break up a code block into two parts (same language in both blocks), I guess I could have used <anything>. I suppose this is the source of all the confusion then.
    – user159834
    Commented Jan 26, 2012 at 21:18
5

You cannot use HTML comments anywhere here on Stack Exchange. Comments such as <!-- filler --> are sometimes used in answers and questions to bypass character limits, but get removed in the final output. Only some HTML is allowed in posts, the rest all gets filtered out.

If you think this should be allowed, you need to fill out a detailed reason why you think you should be able to do this, file a feature request, and let the community downvote it into oblivion.

As per your example, it is finding the <!-- <br> first because the Markdown system doesn't care about your HTML comment. They're not allowed, so it doesn't search for the closing -->. It just sees that <!-- <br> looks like an HTML element, it's not allowed, and thus it gets removed.

Actual HTML source:

<pre><code>The first one &lt;!-- this is the comment --&gt; the second one &lt;!-- this is the second comment &gt;--&gt;
</code></pre>
<hr>
<h3>Result</h3>
<hr>
<p>Result: The first one  the second one --></p>

Notice the lack of actual, physical HTML comments in the source of the document.

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    Look at the edited post. I actually don't want to show the code for comment, I want to actually make it a HTML comment
    – Martin.
    Commented Jan 26, 2012 at 20:30
  • @MatthewRead: I know about &lt;, &gt;, but I actually want to hide the comment from the normal reader
    – Martin.
    Commented Jan 26, 2012 at 20:31
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    Where do you get the idea that HTML comments aren't allowed in posts (as actual, invisible HTML comments)? They are. I use them quite frequently to break up two blocks of code or to leave a note to would-be editors.
    – user159834
    Commented Jan 26, 2012 at 20:46
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    @Madmartigan: Look at the source. Do you see any actual comments in the HTML? They're in the post when you click edit, but they don't show up in the actual output.
    – animuson StaffMod
    Commented Jan 26, 2012 at 20:47
  • You said "the Markdown system doesn't care about your HTML comment. They're not allowed"
    – user159834
    Commented Jan 26, 2012 at 20:48
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    @Madmartigan: They're not. They get removed. That seems very not-allowed-ish to me.
    – animuson StaffMod
    Commented Jan 26, 2012 at 20:49
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    They don't get removed, what do you mean? We're not talking about the actual source code of the page, that's irrelevant. Guys: Are we talking about putting HTML comments in "Comments on a post, like right here" or in the actual post body? Looks very much to me like OP is asking about how this works in an actual SO post.
    – user159834
    Commented Jan 26, 2012 at 20:49
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    @AdamRackis If I understand Martin correctly, he's talking about putting HTML comments in questions and answers.
    – Chris Frederick
    Commented Jan 26, 2012 at 20:50
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    @Chris: yeah, that's it
    – Martin.
    Commented Jan 26, 2012 at 20:52
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    I have no idea why this gets downvoted into oblivion. This is the correct answer.
    – balpha StaffMod
    Commented Jan 26, 2012 at 20:57
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    @balpha - I'm glad I'm not the only one thinking that. This is a bug only by the most narrow, meaningless criteria. Comments are not displayed. Putting certain things in comments breaks things? Then don't do that. There's no use case for you to ever need to do that.
    – Adam Rackis
    Commented Jan 26, 2012 at 20:58
  • @Martin - because the inability to enter something—which is already not allowed—in certain formats isn't really a bug.
    – Adam Rackis
    Commented Jan 26, 2012 at 21:21

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