You don't seem to understand what "canonical" means; it's a certain preferred alternative from among correct alternative forms. Often, the canonical form provides some base definition and the other forms can be understood in terms of equivalence to the canonical form. Do we not understand that a body element is added to the document if it is missing? Is there not a body element in the resulting DOM? 　If so, then the source syntax which has that body can be considered canonical.
Is that really necessary?
> the above broken HTML
As I very clearly stated, it's not broken. It's completely correct, valid HTML. Stick it in a validator if you don't believe me. Yes, I know a lot of people assume otherwise. That's because HTML is only superficially simple but has unexpected irregularities once you dig deeper. This just reinforces my point that HTML isn't the nice neat package that fits well with S-expressions you think it is. The canonical HTML representation of that "broken" HTML is simply the HTML I provided, unaltered, which is not conveniently representable as an S-expression. Please, before trying to reinvent HTML-as-S-expressions, take the time to learn what is and isn't correct HTML. You seem to be assuming the language is simpler than it is and any irregularities are because the sample HTML provided is "broken". This isn't the case.