New principle: How to attach meta-information to a document

[martinthomson]

There are several ways to attach meta-information to documents. There are some trade-offs to understand for each.

• data-* attributes (which make information available to script and somewhat to styling)
• new custom elements (which make information available for presentation and styling, but can also be hidden)
• meta tags in the header
• link relations in the header
• HTTP header (or footer) fields

It might be good to list these out, point out that it is worth seeing if your use case is already addressed by a pre-existing thing for one of these other items, and go through some pros and cons of each.

Related to #213, and which might supersede #213.

[hober]

Not just documents, but also origins and sites. That is, this principle should also call out when to or when not to use /.well-known/ URLs, when or when not to add stuff to Web Manifest, and the like.

[annevk]

Is the audience for this specification authors or web developers? As the former should never touch data-* or custom elements.