What do you mean by "never been specified"? They are in the spec: https://www.w3.org/html/wg/spec/syntax.html#start-tags
> Then, if the element is one of the void elements, or if the element is a foreign element, then there may be a single U+002F SOLIDUS character (/). This character has no effect on void elements, but on foreign elements it marks the start tag as self-closing.