Currently, only A-Za-z characters are valid marks.
Only one mark named "a" (or anything else) may exist at a time, so if one puts a mark "a" in a first document and set another mark "a" in a second document, ELinks will simply replace the former one.
ALL the document marks are always local to the document. I.e. the vim text editor has an extension that makes the capital-letter marks to be global to the whole program and going to such a mark will make it to open the right document. This is not implemented in ELinks _yet_.
Contrary to vim, ELinks doesn't support numbered marks (jumping to the last n documents in history) nor the special "'" mark (jumping to the last mark). Yet.
There is no way to get a listing of all marks set in a document. Yet.