11. A computer program product, encoded on a computer-readable storage device, executable by one or more processors to perform: receiving, concurrently with parsing a structured document, a request from a client to access data in the structured document while the parsing by a computing device is performed serially from a beginning to an end of the structured document, and producing a data object in a memory which is tree-structured and changes as the parsing progresses; in response to the request, and while the structured document is being parsed, determining that the data is not yet available in a portion of the data object by ascertaining a state of the parsing; and when the data becomes available in the data object, and before the parsing has completed, performing the requested access on the portion of the data object, returning any requested data to the client, and deleting the portion of the data object as parsing progresses.
11. A computer program product, encoded on a computer-readable storage device, executable by one or more processors to perform: receiving, concurrently with parsing a structured document, a request from a client to access data in the structured document while the parsing by a computing device is performed serially from a beginning to an end of the structured document, and producing a data object in a memory which is tree-structured and changes as the parsing progresses; in response to the request, and while the structured document is being parsed, determining that the data is not yet available in a portion of the data object by ascertaining a state of the parsing; and when the data becomes available in the data object, and before the parsing has completed, performing the requested access on the portion of the data object, returning any requested data to the client, and deleting the portion of the data object as parsing progresses. 13. The program product of claim 11 where the data object comprises a document object model representing a tree data structure, and where the requested data is a node in the tree data structure.