1. One or more computer-readable media having computer-executable instructions embodied thereon for performing a method of identifying a designated portion of a web page after the web page has been updated, the method comprising: receiving, at a computing device, a selection of a first subsection of the web page; retrieving and storing, at the computing device, a first version of the web page; after a threshold time period, retrieving, from a server hosting the web page, a second version of the web page; generating, at the computing device, a first condensed document-object-model (“DOM”) tree based on the first version of the web page by removing qualifying nodes from a full DOM tree based on the first version of the web page, wherein the qualifying nodes have comparatively less value for mapping a portion of a first DOM tree to a portion of a second DOM tree; generating, at the computing device, a second condensed DOM tree based on the second version of the web page by removing qualifying nodes from a full version DOM tree based on the second version of the web page; and identifying, at the computing device, a second subsection within the second version of the web page that corresponds to the first subsection using the first condensed DOM tree and the second condensed DOM tree.