Web updates

HTML 1dec930b3df9

Restrict the main element to be used once per document Multiple main elements are still allowed if all or all but one have the hidden attribute specified. The main element is also restricted in terms of where it can appear in a document. It can only have html, body, form without accessible name, div, and autonomous custom elements as ancestors. This also slightly alters the definition of the body element as it effectively encompasses all content of a document, except for some metadata. Fixes #100.

Anne van Kesteren