## 1 Introduction  
The presentation and dissemination of interactive 3D content on desktop and mobile web browsers has undergone great advancements during the last few years. This is due to several factors, including: (a) improvement of browsers Web3D capabilities; (b) browsers integration with devices built-in hardware (webcam, microphones, GPS, compass, etc. ); (c) adoption of modern standards (e.g., WebXR); (d) introduction and standardization of 3D formats tailored to interactive web presentation (e.g., gITF). Web browsers are available on virtually all computing devices, thus users can access interactive applications (web-apps) from any device, anywhere, as long as internet (or local network) connection is present. Web applications are also becoming very appealing for the mobile world and especially the Cultural Heritage field (e.g., Museums, Institutions, etc....), due to users not being forced to install any third-party software from stores, nor require additional components to inspect a 3D model or interact with immersive AR/VR experiences. The web represents a great opportunity to build cross-device web-apps, although such opportunity introduces additional challenges related not only to the creation of responsive user interfaces (UIs), but also to the automatic and seamless adaptation of the application to the device. The \"ideal\" web-application should be able to reach every device, automatically adapting its interface and interaction model, thus resulting in a single, liquid product that can be consumed on mobile devices (smartphones, tablets), PC, museum kiosks and immersive VR devices (HMDs). Under this light, there is also a growing need for something more exciting than viewers for mere inspection of 3D objects through a web browser: there is an interest in tools for crafting online Web3D/WebXR experiences, applied 3D games, teaching tools and also collaborative web platforms (like _Mozilla Hubs_) tailored to CH needs and requirements. It is not unusual to visit museums or archaeological sites that offer technological solutions in the form of mobile applications to augment or enrich the visitors' experience using their own devices. In order to consume such interactive experiences, generally visitors have to install applications or third-party software on their smartphones/tablets. From a developer perspective, stores (e.g., Apple store, Google Play, etc.) have precise guidelines and review processes that sometimes can be fairly complex or result in prolonged acceptance times, especially regarding immersive VR applications. Another challenge for developers arises from traditional application development, where the re-use of code between native solution, web-application and immersive VR solution is very poor, thus leading to the multiplication of effort. Regarding cross-device applications, from a design standpoint several challenges arise not only related to the responsiveness of the application (different screen sizes require adaptable user interfaces) but also to the interaction model adopted. For instance, exploring/inspecting a 3D scene through a multi-touch device and through an HMD (immersive VR) require radically different interaction models [1]. This generally leads to the development of different products for different devices, thus creating--once again--fragmentation. Main challenges usually identified in the literature within potentially complex 3D scenes consumed on the web include: (a) network traffic; (b) web-app memory footprint; (c) rendering performance. Immersive computing introduces additional demands and performance requirements for low-latency communication in order to deliver a consistent, smooth and acceptable experience. These have to be taken into strong consideration, especially within the limited resources available on mobile web browsers, compared to desktop counterparts. A big challenge is represented by 3D formats suitable for the web, taking into account state of the art challenges largely addressed in common literature, but also production pipelines of content creators.