An anchor element MUST contain something other than whitespace, or it won't work on many browsers. The image is a clickable imagemap. Citation of book, article, movie, etc. The stuff in such a tag is ignored. Theoretically, a comment can include other HTML tags, but you're much wiser NOT doing this, since many implementations don't support it.
Some implementations restrict comments to a single line.
Where protocol is one of http, gopher, ftp, file, telnet, wais, news, mailto, etc. A fully absolute URL contains a protocol prefix, and a full hostname for external DNS resolution. Encloses the entire form. A list of items to select. The closing tag is optional.