# Page Content Overrides The proxy relies on the original site for substance: both translatable content and server-side services are provided by the original. The proxy relays and processes request-responses between that server and the visitor. PCOs, as they are usually abbreviated, are an exception to this rule. They are “virtual” pages that are either created completely from scratch (and thus may not even have a counterpart on the original) or they override URLs that are accessible on the original server with wholly custom content over the proxy. If a visitor requests an URL over the proxy that has an associated PCO, then the corresponding request is not sent to the origin (even if the URL is made unique by query parameters) and the PCO is served instead. A PCO, in a similar fashion to responses from the original site, will go through the translation pipeline. This bears repetition: **the contents of a PCO can be extracted and translated**. ## Headers ### Content Types The source need not be HTML: any custom content-type can be entered (as long as it is a textual type), such as `text/xml` or `application/javascript` along with customized cache headers and status codes. Of the many content types that can be used, the behavior of `text/html` is special in an important way: CSS and JS page modifiers on the project will be **injected** into such PCOs. The corresponding `