The production for Attribute refers to
AttValue, which is inconveniently defined
elsewhere.
But if you look closely, note that you can have entity references in attribute
values.
In fact, this turns out to be very useful. In the document you are now
reading, there are a lot of hyperlinks into the XML spec; these are
done with an ordinary href attribute, for example:
<x href='&spec;id(sec-starttags)'>
Then we can use the official copy of the spec at the W3C site, or a local copy
on disk, just by changing the definition of the spec attribute.
Also it saves a lot of typing and disk space.