1. A method comprising: generating, by a system of one or more computers, a first data structure based on a web page, where the first data structure includes nodes corresponding to text that will be visually displayed from the web page and nodes corresponding to text that will not be visually displayed from the web page when the web page is rendered for display by a client device; generating, by the system of one or more computers, a second data structure for the web page based on the first data structure, the second data structure including nodes that correspond to text that will be visually displayed when the web page is rendered for display by the client device; comparing, by the system of one or more computers, nodes corresponding to text of the first data structure with nodes corresponding to text of the second data structure to identify the nodes corresponding to text that will not be visually displayed from the web page when the web page is rendered for display by the client device; and generating, by the system of one or more computers, weighting factors for the nodes corresponding to text that will not be visually displayed from the web page when the web page is rendered for display by the client device based on the nodes corresponding to text that will not be visually displayed from the web page.