The WHATWG works on a number of technologies that are fundamental parts of the web platform. They are organised somewhat arbitrarily based on the preferences of those editing the standard for those technologies.
The Compatibility Standard describes a collection of non-standard (and often vendor-prefixed) CSS properties and DOM APIs that web browsers need to support for compatibility with the de facto web.
The Compression Standard provides APIs for compressing and decompressing bytes using a set of the most common compression algorithms.
The Console Standard defines APIs for console debugging facilities.
The DOM Standard defines the core infrastructure used to define the web.
The Encoding Standard defines how character encodings work on the web.
The Fetch Standard defines the networking model for resource retrieval on the web.
The File System Standard defines infrastructure and an API for file systems.
The Fullscreen API Standard defines how web pages can take over a user's entire screen (at the user's request), e.g., for gaming or to watch a video.
The HTML Standard is a kitchen sink full of technologies for the web. It includes the core markup language for the web, HTML, as well as numerous APIs like Web Workers, localStorage, etc.
The Infra Standard aims to define the fundamental concepts upon which standards are built.
The MIME Sniffing Standard defines algorithms used to determine the type of resources.
The Notifications API Standard provides an API to display notifications to alert users outside the context of a web page.
The Quirks Mode Standard describes behaviours in CSS and Selectors that are not yet defined in the relevant specifications but that are nonetheless widely implemented.
The Storage Standard defines an API for persistent storage and quota estimates, as well as the platform storage architecture.
The Streams Standard provides APIs for creating, composing, and consuming streams of data that map efficiently to low-level I/O primitives.
The Test Utils Standard defines internal APIs for automating testing of web platform features implemented in web browsers.
The URL Standard defines the infrastructure around URLs on the web.
The URL Pattern Standard provides a web platform primitive for matching URLs based on a convenient pattern syntax.
The Web IDL Standard defines an interface definition language, Web IDL, that can be used to describe interfaces that are intended to be implemented in web browsers.
The WebSockets Standard provides APIs to enable web applications to maintain bidirectional communications with server-side processes.
The XMLHttpRequest Standard defines the networking API exposed to scripts on the web.
The WHATWG also works on a number of ideas that aspire to become standards one day.