Distinguish our aspirations from what we have already operationalized

[dontcallmedom]

Following up on #78 (comment) - #78 adds sustainability as a goal in the same list as accessibility or internationalization. We have well-established practices and processes for the latter that comes with relatively good clarity of what it means to make the Web accessible or ready for I18N, but we're at the very beginning of understanding what "sustainable" means for the Web.

I think we should find a way to make it more clear what part of the vision is anchored on paved cowpaths, compared to less well-defined aspirations.