Propose to change

"The max and value IDL attributes must reflect the respective content attributes of the same name. When the relevant content attributes are absent, the IDL attributes must return zero."

to instead read something along the lines of:

"The max and value IDL attributes must reflect the respective content attributes of the same name. When the value content attributes is absent, the value IDL attribute must return zero.  When the max content attribute is absent, the max IDL attribute must return one."

The reason is that in a previous paragraph, the spec says "Otherwise, if the element has no max attribute, or if it has one but parsing it resulted in an error, or if the parsed value was less than or equal to zero, then the maximum value of the progress bar is 1.0."

It makes sense that if the progress bar is using 1.0 as its maximum value then this should be the value returned by the IDL attribute if the content attribute is missing. Without this, code that consumes the IDL attribute has to also include the logic above from the spec to know how the control will behave.