security Security matters are not considered here. If this concerns or bothers you, please stop reading.
display server A display server is something which provides the service of display to something else. That “something else” is a client program. The terminology is sometimes confusing because the user interacts with the client program via the display server, the latter being a program running on the user’s device. This setup is very useful when one needs to have a graphical experience with a program running on a remote system.
remark However, the main effect is to confuse new users with the ambiguous terminology. This is the only situation of which I know where the user controls a client thing through a server thing.
browser The web browser is a case of arrested development. It’s not a display server, but it’s where web applications display themselves. Much has been written about “the browser as operating system”. This sounds clever, but does not really provide much insight, and is only true for a narrow sense of the term “operating system”.
browser apps these occupy an anonymous ground running in the browser on the desktop or on a mobile device.
universality Given that digital content is made up of many formats, we need a tool to be able to (at least) render all of them — to literally browse collections of local and remote content, and to do so without having to install another application on one’s device.
renderer A universal document renderer is able to gain new abilities to display newly-encountered media types (such as audio), and newly-encountered encoding formats (such as MP3, OGG, JPEG, MOV, …), and does so without interrupting the user, except perhaps for confirming the addition of the new ability.
content types A universal document renderer has a bootstrap architecture, and broadly speaking, is little more than a framework for rendering plugins of various media types. This demotes the DOM / HTML / CSS combination from its current triumvirate status to be that of “just another content type”, and permits new expressions of interactive media.
transfer protocols HTTP could also lose its place as the dominant transfer protocol. URL notation is sufficient to enable the multi-protocol handling central to the bootstrapping nature of the system. HTTP remains as a basic transport for “DOM / HTML / CSS” content and renderer implementations.
implementations Code for rendering lives in repositories on the network. Universal document renderers are directed by content type metadata to fetch implementations of decoders for newly-encountered content types from these repositories. Once fetched, they remain cached locally by the framework until a new version is available, or they are no longer required and may be deleted. Universal document renderers may also check repositories periodically to ensure they have the desired (usually the latest) versions.
conclusion Browsers are lacking in flexibility to be classified as universal document renderers. Browsers have not gained the capability of rendering all possible document formats. In practice, the dominant document format is DOM via HTML and CSS, with PDF closely behind. Video (and to a lesser extent audio), have their own codecs but this is decided mainly by the main video sites and browser producers.
However, as browsers have evolved, they have effectively become a possible successor to display servers such as X11. This may be attributed to the efforts of developers of web frameworks, more than to those of browser authors.
The “next display server” is a universal document renderer, incremental in nature, and invisible in action. A display server is a system-level concept. A universal document renderer is a user- and application-level concept.