Add more links to related specifications

[JKingweb]

In parsing element classes for microformats the specification assumes how to split the the class attribute value into tokens is known to the reader, but there is a reference which could be added for those environment which don't have a classList property for HTML elements:

https://infra.spec.whatwg.org/#split-on-ascii-whitespace

Similarly, the specification makes reference only in vague terms to collapsible whitespace:

'spaces' in the algorithm refers to HTML collapsible blank spaces of any kind, whether ' ' or linefeed, carriage return, etc.

Presumably this reference would do:

https://infra.spec.whatwg.org/#strip-leading-and-trailing-ascii-whitespace