"--" notation for modifiers breaks XML/XHTML when inside comment

[pbb72]

The following causes a fatal error in XML/XHTML, and will break any page interpreted as XML:

<!-- div class="block block--mod">...</div> -->

This is because the double dash is interpreted as a comment delimiter, when inside an SGML instruction (<! ... >).

While this problem occurs for any use of double dashes, it's regrettable when these are introduced as part of a system.