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 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 Sockets, Web Workers, localStorage, etc.
The Infra Standard aims to define the fundamental concepts upon which standards are built.
The DOM Standard defines the core infrastructure used to define the web.
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 Notifications API Standard provides an API to display notifications to alert users outside the context of a web page.
The Encoding Standard defines how character encodings work on the web.
The URL Standard defines the infrastructure around URLs on the web.
The Fetch Standard defines the networking model for resource retrieval on the web.
The MIME Sniffing Standard defines algorithms used to determine the type of resources.
The XMLHttpRequest Standard defines the networking API exposed to scripts on the web.
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 Console Standard defines APIs for console debugging facilities.
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. These streams are designed to map efficiently to low-level I/O primitives, and allow easy composition with built-in backpressure and queuing. On top of streams, the web platform can build higher-level abstractions, such as filesystem or socket APIs, while at the same time users can use the supplied tools to build their own streams which integrate well with those of the web platform.
The Books Standard defines CSS features for book publishing and high-quality printing of web pages.
The Figures Standard defines CSS features for floating elements around pages and columns.
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.
For more details, please see the relevant FAQ entry.