1.1.4 The !DOCTYPE statement

Checkpoints in this section:

• 3.2 Create documents that validate to published formal
  grammars. [Priority 2]

Validating to a published formal grammar and declaring that validation at
the beginning of a document lets the user know that the structure of the
document is sound. It also lets the user agent know where to look for semantics
if it needs to. The W3C Validation
Service validates documents against a whole list of published
grammars.

It is preferable to validate to W3C grammars. Refer to the Technologies
Reviewed for Accessibility.

1.1.5 The
LINK element and navigation tools

Checkpoints in this section:

• 13.9 Provide information about document collections (i.e.,
  documents comprising multiple pages.). [Priority 3]

Content developers should use the LINK element and
link types (refer to [HTML4], section 6.12) to describe
document navigation mechanisms and organization. Some user agents may
synthesize navigation tools or allow ordered printing of a set of documents
based on such markup.