Build web proxy that can support difficult / javascript-heavy websites

We are looking for an expert developer who can help us work on a difficult project. We are looking to build a simple web proxy, that works for difficult websites. It should be able to do the following: - Load third-party websites from our domain (i.e. [login to view URL]) - Depending on which webpage of the proxied third-party website the user is on, add our code onto the page (i.e. add a button on the 'product' page of [login to view URL]) - *It must not obstruct cookies, or mess with the proxied website's javascript.* - You may use whatever programming language you want, and whatever open-source script you'd like - Only needs to work for 10 pre-determined websites (i.e. [login to view URL], [login to view URL], etc) This is especially challenging because, for example, some of [login to view URL]'s webpages load page elements dynamically via javascript, and it does not work using most open-source proxy scripts. The challenge is to make sure it works seamlessly. For example, the proxy must work for this webpage: [login to view URL] Most web proxy websites are not able to make the URL above work (i.e. [login to view URL]), while I can only find one web proxy that can make it work: [login to view URL] Please try for yourself.