Example embodiments relate to measuring compatibility of viewers. An example method may include accessing a document object model (DOM) generated by an anchor document viewer in response to rendering document code. The DOM may include multiple DOM elements that correspond to graphical elements displayable by the anchor document viewer. The method may include, for each of at least one of the multiple DOM elements, allowing a user to provide an element definition that indicates how important it is that the corresponding graphical element displays as it does in the anchor document viewer.