1. A method comprising: receiving, by a computing system, a request for a web page; sending, by the computing system, a first response portion for generating the requested web page to a client computing device in response to the request, the first response portion comprising: a structured document for use by the client computing device to render the requested web page and comprising: markup-language code that comprises one or more place-holder code segments; a code segment for implementing a page-assembling process that comprises a function library; and one or more first resources or one or more first references to first resources, wherein sending the first response portion comprises sending the code segment comprising the function library; generating, by the computing system, one or more second response portions each for generating a corresponding portion of the requested web page, each second response portion comprising: markup-language code and one or more of one or more second portions of content to be rendered by the client computing device, one or more second resources, or one or more second references to second resources to be downloaded; and a function call to a library function in the function library of the page-assembling process, wherein the generated markup-language code comprises the function call; and sending, by the computing system, the second response portions to the client computing device; wherein the page-assembling process is configured, responsive to a function call corresponding to a respective second response portion, to: identify a place-holder identification (ID) parameter in the function call; access a model representation, generated by a client rendering application, of the structured document that comprises one or more place-holder code segments, each place-holder code segment being represented as a place-holder node in the model representation, each place-holder code segment and respective place-holder node having a corresponding place-holder ID parameter; identify a place-holder node in the model representation by matching the place-holder ID parameter in the function call to the corresponding place-holder node having a matching place-holder ID parameter; and insert, or cause to be inserted, the second portions of content, resources, or calls to resources in the second response portion corresponding to the function call into the model representation at a location in the model representation determined based on the corresponding matching place-holder node.
6. The method of claim 1 , wherein each second response portion comprises an identifier parameter that is usable by the page-assembling process to identify the corresponding place-holder code segment in the structured document.