HTML comments are not metadata but, are typically found in the <head></head> section of web pages. HTML comments can be utilized anywhere within your documents HTML structure.
<!-- HTML Comments (treated as HTML markup) -->
There has been a myth that has perpetuated over the years where keywords and keyword phrases listed inside HTML comments tags would add a boost to the overall relevancy of the page. This is not true based on numerous tests we've performed during the years 2002, 2003 and 2004.
fuente:
http://www.seoconsultants.com/meta-tags/
Según mi experiencia no son relevantes al igualq ue esta gente yo he hecho pruebas y no, no ha funcionado, ahora si fuera cierto, tomen el lado lógico imaginense el monto de spam que eso significa para cualquier buscador