`—` or `—` is there any difference in HTML output?

Question

— or —

Is there a difference between these? Is one better-supported than the other?

Answer 1

SGML parsers (or XML parsers in the case of XHTML) can handle — without having to process the DTD (which doesn't matter to browsers as they just slurp tag soup), while — is easier for humans to read and write in the source code.

Personally, I would stick to a literal em-dash and ensure that my character encoding settings were consistent.