Through the past few years, in-browser streaming of audiovisual content has become a commodity. Due to the diverse nature of possible audiovisual applications, there is often a need for accompanying descriptors and other metadata, such as semantic annotations, captions, etc. The metadata need to be sent in a timely fashion along with the multimedia content. The use of XML in such cases is common, and the usual approach for in-browser transmission of such data is via AJAX. Even though AJAX can be sufficient for many services, there is consideration for few offline or live scenarios. With MP4Box and MP4Box.js we are able to synchronously stream and consume XML and multimedia data, packaged in MP4 containers, with a standard browser. Accompanying XML documents can be transmitted as a whole, or progressively (in fragments). In this paper, we define the use-cases for this technology, analyze the requirements and present the mechanisms of MP4Box and MP4Box.js for XML end-to-end transmission inside the browser.