Module invalidation

Different bits of code related to invalidating style.

Modules

• element
  Invalidation of element styles due to attribute or style changes.
• media_queries
  Code related to the invalidation of media-query-affected rules.
• stylesheets
  A collection of invalidations due to changes in which stylesheets affect a document.
• viewport_units
  Invalidates style of all elements that depend on viewport units.