Get external resources from html code

I'm trying to build a simple web proxy to retrieve some webpage content and proxy all further requests. i can do that quite well with javascript requests by overriding the native functions like xhr , appendchild , .... but for the html resources like <img src...> or <script src..> ... i cant do nothing , so my question is if there's a way to "render" the html via javascript and extract all the resources and proxy the url or should i do that server side?