1. A computer system comprising: at least one processor configured to execute instructions stored on a non-transitory computer-readable medium to execute a browser application to provide a browser interface; a rendering engine configured to cause the at least one processor to execute, within an execution environment, a page script of a page to be rendered within the browser interface, the page script configured to interact with a page model to render the page within the browser interface; an extension manager configured to cause the at least one processor to execute an extension file which modifies a functionality of the browser application in association with the rendering of the page, including detecting a content script associated with the extension file which, during execution, interacts with the page model; and a content script manager configured to cause the at least one processor to evaluate the page prior to allowing execution of the content script associated with the extension file by the rendering engine, wherein the content script manager is configured to prevent the content script from accessing the execution environment when the page fails the evaluation; and wherein the content script manager is configured to allow the content script to access the execution environment when the page passes the evaluation.