1. A computer-implemented method for determining positions at which a plurality of content components within original content of a digital publication are rendered by an independent layout engine, comprising: producing modified content by modifying visual attribute values of the plurality of content components of the original content, the content components of the original content being described by markup tags, the modifying comprising at least one of modifying the markup tags and adding additional markup tags associated with the content components, wherein the modifying does not modify layout positions of the content components of the original content when rendered by the layout engine; associating the modified visual attribute values with the content components to which they correspond; producing a modified rendering by rendering the modified content to an offscreen graphics buffer using the layout engine; identifying changes in pixel values over the modified rendering; identifying positions of regions within the modified rendering of the offscreen graphics buffer that have the modified visual attribute values, based on the identified changes in pixel values; and associating the identified positions with content components based on the associations of the modified visual attribute values with the content components.