I did find this in the validator.w3.org error docs... the same validator you use.
In fact, unless you have extreme file size requirements it is a very very good idea to always put quote marks around your attribute values. It is never wrong to do so, and very often it is absolutely necessary.
I've been in web dev for a long time and I've never heard of quotes being invalid markup. Can you post exactly where it says this in the spec?
I did find this in the validator.w3.org error docs... the same validator you use.
validator.w3.org/docs/errors.html#...