1. A computer-implemented method for aggregating rules that define values, comprising: receiving a first declaration of a first style rule of a cascading style sheet in a mark-up language document, wherein the first declaration of the first style rule includes a first selector declaration and a first style definition associated with the first selector, wherein the first style definition includes first properties indicated with first values for rendering the first properties; receiving a second declaration of a second style rule of the cascading style sheet in the mark-up language document, wherein the second declaration of the second style rule includes a second selector declaration and a second style definition associated with the second selector, wherein the second style definition includes second properties indicated with second values for rendering the second properties, wherein the first declaration of the first style rule and the second declaration of the second style rule form a prioritized hierarchy in the mark-up language document; displaying, within a user interface, a property grid, wherein the property grid includes the first and second style rules in association with a property list that identifies a list of properties associated with the values of the style rules for the first style rule and for the second style rule; receiving a selection, within the user interface, to aggregate the first values of the first style rule and the second values in the second style rule to generate an aggregate property grid, wherein conflicting values of properties indicated in the first style rule and the second style rule are merged according to the prioritized hierarchy of the mark-up language document, wherein the values of the second style rule in the prioritized hierarchy overrides conflicting values of the first style rule of the prioritized hierarchy; and displaying, within the user interface, the aggregate property grid, wherein the aggregate property grid includes a display property list that identifies a list of properties for rendering according to the prioritized hierarchy of the mark-up language document, wherein values in the first style rule in the prioritized hierarchy that are overridden by values of the second style rule are marked in the aggregate property grid with an override indicator, wherein overridden values are selectable to negate the override.