In 1987 the XWindows consortium was formed at MIT (http://en.wikipedia.org/wiki/X_Window_System). X Windows was this weird, powerful thing that allowed an application and its UI to be split across a network. It was very prominent in the GUI wars back in the days when "Workstations" where from companies like Apollo and Sun.
In 1994, W3C consortium was formed at MIT. Web browsers begin to take over the client side of the planet.
Meanwhile, Linux - with the X Windows System built into it - is steadily taking over the server side of the planet.
In 2013, HTML5 is central to a new era of Web - namely dynamic assembly of "pages" with lots of Javascript being executed client side under the control of the server which is sending over the instructions. The Javascript has access to a canvas element and can basically scribble on the screen pixel-by-pixel. A new era in rich-web-apps is possible.
Sit at, say, an Ubuntu laptop. You are most likely running an X Windows based GUI. It is perfectly capable of doing client/server apps where the server side controls what appears on the screen, down to the pixel level, using the X network protocol.
You are almost certainly not doing that however. You are most likely running a browser on top of X. It is most likely using HTML5 to do what the X system could do itself, namely, remote screen control from the server side.
So server generated HTML contains Javascript which ultimately makes X calls, rather than the server side directly doing the X calls.
Pretty weird. There appears to be umpteen unnecessary layers involved in all this to me. Part of me thinks that there is an alternative universe where we are all using what where confusing known as "X Servers" on our desktop machines, rather than browsers.
IT sure is weird.