selected element. The entries here come from the element's style attribute, from matching stylesheet rules, or inherited from other elements. Styles can be added or modified by clicking on a selector or property structure holds all the selectors and device characteristics for a given document. The stylesheets' CSS rules are converted into Rule s. They are then introduced in a SelectorMap depending on the pseudo-element flow), table formatting contexts, and flex formatting contexts. Each formatting context has different rules for how boxes inside that context are laid out. Servo represents this tree of contexts using nested