Addresses

• [css-display-3] Clarify whether the initial containing block establishes a block formatting context #7207
• [CSS2] Clarify how the root element is laid out. #6480
• [css-display] "root element" is undefined #7786

by

• defining "root element" as a term in the glossary
• defining that the root element's box always creates an independent formatting context
• adding a subheading over the paragraph about root element special behavior
• defining the ICB to establish a block formatting context so that the root element's box is fully defined to be a block-level box

This last bit is implemented (as demonstrated by float) in WebKit and Blink; Firefox seems to have special layout for the root that doesn't honor float, but is otherwise like block layout. We could say that float is ignored or whatever, but definitely most of the behavior (margins, alignment, sizing) matches block layout.