Question

<!-- here is some comment --
                            ^
                            |
                    what can be here apart from '>'?

XML seems not to like ‘–‘ inside comments. I read somewhere that ‘–‘ switchs some modes inside <! ... > thing, but <!-- -- -- --> (even number of --s) seem to be invalid too. If it is some historic feature, what is “pro” part of it? (“contra” part is inability to have -- in comments).

What is the reason of complicating comment processing by not making just ‘–>’ end of comment and allowing ‘–‘ inside?

Answer 1

From the standards document:

http://www.w3.org/TR/REC-xml/#sec-comments

[Definition: Comments may appear anywhere in a document outside other
markup; in addition, they may appear within the document type
declaration at places allowed by the grammar. They are not part of the
document’s character data; an XML processor may, but need not, make it
possible for an application to retrieve the text of comments. For
compatibility, the string ” — ” (double-hyphen) must not occur within
comments.] Parameter entity references must not be recognized within
comments.