In practice this will be the 'agent' level rather than the 'author' level, given
the SetParsingMode call immediately after creating the stylesheet. This change
aligns behavior with CSSStyleSheet::ReparseSheet.
One of the annoying things about sharing algorithms on the superclass here is that
certain members live on StyleSheet and others live on StyleSheetInfo (a situation
resulting from the split between CSSStyleSheet and CSSStyleSheetInner). This allows
us to write general algorithms on StyleSheet that touch members on StyleSheetInfo
without paying the price of virtual dispatch.
Bobby Holley
Cameron McCormack