An example embodiment may involve receiving, by a web server device and from a client device, a request for web content. The example embodiment may also involve determining, by the web server device, that a web document includes a script containing a synchronous client-side function call matching pre-determined criteria. The web content may be at least in part derivable from the web document. The example embodiment may also involve executing, by the web server device, the synchronous client-side function call to obtain output data. The example embodiment may also involve modifying, by the web server device, the web document to include the output data in a data structure associated with the synchronous client-side function call. The example embodiment may also involve transmitting, by the web server device and to the client device, the web document as modified.