1. A method of pre-fetching resources, comprising: scanning an HTML document to discover a script by an HTML document scanner executing in a processor of a computing device; invoking a script execution engine and spawning a sandboxed script execution engine in response to the HTML document scanner discovering the script, wherein: the script execution engine: commences compiling and executing the discovered script on the computing device concurrent with execution of the HTML document scanner and the sandboxed script execution engine; and the sandboxed script execution engine: executes on the computing device concurrent with the execution of the HTML document scanner and the script execution engine; selects operations in the discovered script that are relevant to discovering external resources; and executes only the selected operations to discover resources that are not expressly referenced in the discovered script or in the HTML document; pre-fetching the resources by the sandboxed script execution engine issuing a request to a pre-fetcher to download the discovered resources that are not expressly referenced in the discovered script or in the html document, wherein pre-fetching the resources includes the sandboxed script execution engine pre-fetching a resource in parallel with the script execution engine executing a different script and concurrent with an execution of a browser operation; and discarding results generated via the sandboxed script execution engine executing the selected operations of the discovered script.