Parsoid/Todo:Semantically insignificant whitespace round-tripping

We need clean diffs, including semantically insignificant whitespace such as extra spaces between attributes in an HTML tag.

Possible solutions:

Tracking all semantically insignificant whitespace in rt information.
- Advantage: works with any client
- Disadvantage: a lot of work and rt info / code bloat; high cost/benefit
Selective serialization of modified DOM parts
- Advantage: better cost/benefit ratio
- Disadvantage: adds a burden on clients to mark modified parts of the DOM