21. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: requesting, by a client device and over a network, a resource of a document from a first source; before receiving the requested resource, requesting, by the client device and over the network, metadata for the document from a second source that is different from the first source; receiving, by the client device, the requested metadata for the document from the second source, wherein the received metadata comprises data indicating characteristics of layout or formatting of the document; receiving, by the client device, the requested resource of the document from the first source; providing, for display and by the client device, a first representation of the document based on the received metadata from the second source; and after providing, for display, the first representation of the document based on the metadata from the second source: generating, by the client device, a second representation that combines portions of the first representation with additional portions of the document; and providing, for display and by the client device, the second representation that combines portions of the first representation with the additional portions of the document.
22. The system of claim 21 , wherein requesting metadata for the document from the second source that is different from the first source comprises requesting metadata for the document from a second source that is not identified by resources of the document.