HTML attribute with no value

In the f2f, we said:

ubi syntax is ubi=”ubi”|””|”off”. The “ubi” and empty string values are
equivalent, and mean that bidi isolation is on for the element.


I am confused about why we needed two values meaning "on", and if so, why
neither one of them is named "on".

The crux of my question is what, exactly, does the HTML spec say about an
attributes with no value, e.g. <span foo>? Is it always equivalent to <span
foo=foo>, or to <span foo="">, or something else?

If <span foo> is equivalent to <span foo=foo>, I do not see why we need an
empty string value.

If, on the other hand, <span foo> is equivalent to <span foo="">, I
understand why we need an empty value as well as an equivalent non-empty
value, but I do not understand why the latter has to be the meaningless
"ubi". I would prefer ubi=”on”|””|”off”.

Aharon