15. A method comprising: generating, by a system of one or more computers, a document object model (DOM) tree for a first web page, where the DOM tree includes nodes corresponding to text that will be visually displayed and nodes corresponding to text that will not be visually displayed when the first web page is rendered for display by a client device; generating, by the system of one or more computers, a render tree based on the DOM tree, the render tree including nodes that correspond to text that will be visually displayed by the client device when the first 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 DOM tree with nodes corresponding to text of the render tree to identify the nodes corresponding to text of the DOM tree that will not be visually displayed when the first web page rendered for display by the client device; and creating, by the system of one or more computers, a hidden text data index based on the DOM tree and the render tree, the hidden text data index identifying nodes corresponding to text of the DOM tree as hidden that will not be visually displayed when the first web page is rendered for display by the client device and including a weighting factor associated with the identified nodes.
16. The method of claim 15 , further comprising: generating, by the system of one or more computers, a second document object model (DOM) tree for a second web page, where the second DOM tree includes nodes corresponding to text that will be visually displayed and nodes corresponding to text that will not be visually displayed when the second web page is rendered for display by the client device; generating, by the system of one or more computers, a second render tree based on the second DOM tree, the second render tree including nodes that correspond to text that will be visually displayed by the client device when the second 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 second DOM tree with nodes corresponding to text of the second render tree to identify the nodes corresponding to text in the second DOM tree that will not be visually displayed when the second web page is rendered for display by the client device; and updating, by the system of one or more computers, the hidden text data index based on the second DOM tree and the second render tree, the hidden text data index identifying nodes corresponding to text of the second DOM tree as hidden that will not be visually displayed when the second web page is rendered for display by the client device and including a weighting factor associated with the identified nodes corresponding to the text of the second DOM tree.