1. A computer, comprising: a processor; and a non-transitory computer readable medium comprising instructions that, when executed by the processor, cause the computer to perform operations comprising: parsing code of a markup document using a browser, the code using calls to an application programing interface (API) to specify content to be rendered and the code further specifying a proxy object; creating the proxy object by interpreting the code of the markup document; invoking a runtime environment by interpreting the code of the markup document; updating a state of the proxy object based on a call to the API, the call initiated based on the browser using the code of the markup document, the call to the API native to the browser and non-native to the runtime environment; determining that the state of the proxy object has changed; and based on determining that the state of the proxy object has changed, invoking a command of the runtime environment to provide an additional graphical element or additional functionality in content rendered by the browser, wherein updating the state of the proxy object by interpreting the code of the markup document and then invoking the command based on the change in the state of the proxy object enables the runtime environment to respond to the code in the markup document that uses a property, method, or behavior that is native to the browser but non-native to the runtime environment.