Remove parse error for dashes in comments <!-- -- -->

[zcorpan]

It has been 10 years since SGML-style comments were killed.
http://ln.hixie.ch/?start=1137799947&count=1

Reasons it's a parse error:
https://lists.w3.org/Archives/Public/public-whatwg-archive/2006May/0038.html

This has two benefits:

1. Previously non-conforming syntax won't be made conforming, so pedantic HTML 4 implementations won't break with conforming HTML5 comments.
2. Conforming HTML5 documents can be serialized as conforming XHTML5 document without data loss even when preserving comments. (Strictly speaking in XML processing it is legitimate to drop comments in the bit bucket, so in that sense, dropping comments would not be significant data loss. :-)

(Plus, it was helpful for authors to get a hint from the validator if their page was broken in legacy Firefox that implemented SGML-style comments.)

I think these reasons are no longer relevant. The XML processing case is adequately covered by https://html.spec.whatwg.org/multipage/syntax.html#coercing-an-html-dom-into-an-infoset and nobody cares about what was valid HTML4, and nobody cares about pre-2006 Firefoxes.

So I suggest we make this be allowed:

<!--------------------------------->

cc @hsivonen @sideshowbarker

[sideshowbarker]

Yes please let’s drop this requirement. We have plenty of evidence that it causes problems for authors without actually solving any problems for them:

• http://stackoverflow.com/questions/31701253/how-to-fix-consecutive-hyphens-did-not-terminate-a-comment-error
• https://twitter.com/search?f=tweets&vertical=default&q=Consecutive%20hyphens%20did%20not%20terminate%20a%20comment&src=typd

(BTW I dunno what BEM is but I assume it must be some library or template thing or something that uses -- in comments.)

[zcorpan]

Currently --! in a comment is a parse error, but I don't see any reason for that to be the case:

<!-- --! -->

(But <!-- --!> should still be a parse error because it's a bogus way to close a comment.)

We need to introduce something new to make sure that "nesting" of comments is still caught by conformance checkers, so that this is still be a parse error:

<!-- <!-- foo --> -->

[hsivonen]

We have plenty of evidence that it causes problems for authors without actually solving any problems for them

This is persuasive.

But...

We need to introduce something new to make sure that "nesting" of comments is still caught by conformance checkers

This is persuasive in the other way even if one didn't care about XHTML serializability.

I'd prefer us not tweaking this stuff anymore.

Sorry, but is there any particular reason not allowing -- in comments is problematic? I can see it barely causing any trouble. I’m not sure that four tweets are a good enough reason to make this change.

[sideshowbarker]

https://checker.html5.org/ now experimental support conforming to the spec text in the PR #1356 branch.

Also, sorry if I misunderstand the situation, but, wouldn’t this, besides deviating HTML from XML, also deviate WHATWG’s HTML from W3C’s HTML? I strongly oppose against this change.

[sideshowbarker]

Also about the https://checker.html5.org/ support as #1356 (comment) notes, the new messages emitted:

Saw <!-- within a comment. Probable cause: Nested comment (not allowed).

Error: --!> found at end of comment (should just be -->).

[sideshowbarker]

Note also that you can use https://checker.html5.org/parsetree/ to view resulting parse tree.

[zcorpan]

Hello @Zambonifofex. I don't see any new information in your comments. You are free to think that this is not an important problem to solve, though @sideshowbarker who currently maintains checker.html5.org seems to think otherwise. If you have any reasons for why this is a bad change then now is a good time to put them forward. I'm certainly open to change my mind here if there is new information we have not yet considered.

Any change we make is going to be a change compared to W3C HTML. That is not in itself an argument against the change.