Network connectivity represents a major asset for mobile devices: the Web is an immense store of content, as well as an almost endless source of processing power, overcoming two of the limitations of mobile devices. The Web platform is growing a number of APIs that facilitate establishing network connectivity in different contexts.
Likewise, the opportunity offered to deploy real-time communication services on the Web is another step towards the unification of the telecommunication and Web ecosystems.
XMLHttpRequest (the basis for Ajax development) is a widely deployed API to load content from Web servers using the HTTP and HTTPs protocol: the W3C specification (formerly known as XMLHttpRequest Level 2) was meant to document the existing deployed API (with the ability to make requests on servers in a different domain, programmatic feedback on the progress of the network operations, and more efficient handling of binary content). That work is now done in the WHATWG. The WHATWG Fetch API also provides a more powerful Promise-based alternative.
By default, browsers do not allow to make request across different domains (or more specifically, across different origins, a combination of the protocol, domain and port) from a single Web page; this rule protects the user from having a Web site abusing their credentials and stealing their data on another Web site. Sites can opt-out of that rule by making use of the Cross-Origin Resource Sharing (CORS) mechanism, opening up much wider cooperation across Web applications and services. Note the W3C version of the CORS specification does not reflect current implementations and will be obsoleted. The CORS mechanism is now a core part of the on-going Fetch specification in the WHATWG.
XMLHttpRequest is useful for client-initiated network requests, but mobile devices with their limited network capabilities and the cost that network requests induce on their battery (and sometimes on their users bill) can often make better use of server-initiated requests. The Server-Sent Events API allows triggering DOM events based on push notifications (via HTTP and other protocols).
The WebSocket API, built on top of the IETF WebSocket protocol (RFC6455), offers a bidirectional, more flexible, and less resource intensive network connectivity than XMLHttpRequest.
Of course, an important part of using network connectivity relies on being able to determine if such connectivity exists, and the type of network available. The HTML5 onLine DOM flag, and its associated change event, online , signals when network connectivity is not available to the Web environment (the flag is unreliable to assert that the Web environment is online).
The Resource Timing API offers to measure precisely the impact of the network on the time needed to load various resources, offering another approach to adapt a Web app to its network environment.