A device, system and method are provided for managing memory for rendering webpages and other structured documents that contain multiple regions. A backing store is created in memory for storing rendered document content. A main region of the structured document is rendered for display, divided into a set of tiles, and stored in the backing store. A subregion of the document is rendered and stored as tiles in the same backing store as well. At least a portion of the tiles for the main region and subregion intersecting with corresponding viewports are outputted to a display. When an active one of the viewports is changed and additional content of the document is to be rendered for display, tiles in the backing store used to store rendered but undisplayed data for the inactive viewport are released to store new rendered content for the active viewport.