That is not how I read that....

By default, SGML requires that all attribute values be delimited using either double quotation marks (ASCII decimal 34) or single quotation marks (ASCII decimal 39).

Seems to indicate to me pretty clearly that quotes (either single or double) are required.

In certain cases, authors may specify the value of an attribute without any quotation marks. The attribute value may only contain letters (a-z and A-Z), digits (0-9), hyphens (ASCII decimal 45), periods (ASCII decimal 46), underscores (ASCII decimal 95), and colons (ASCII decimal 58). We recommend using quotation marks even when it is possible to eliminate them.

Now this says that you can omit quotation marks (but it still recomends that you keep them in), but only in certain situations situations. Note that the comma is not one of the characters that is allowed in a non-quoted string.