HTML and CSS Reference
In-Depth Information
Widgets come in many varieties and are rarely harmful. hey run within
the browser's security setup and are generally isolated from your computer's
ile system. However, they can cause trouble if they are not well written. he
problems include messing up the display of a web page, using up too much of
the browser resources, or even causing a browser to crash.
Any stand-alone computer application or sotware program that exchanges
information over the Web (Twitter clients, for example) is a user agent. So are
the automatic sotware update programs that come with computer operating
systems. So is the online Help feature of Microsot Word or, for that matter, an
Xbox, Nintendo, or PlayStation game console. Many of the apps on a modern
smartphone are user agents, sending requests to web servers and using the
returned information to do something useful or keep you informed.
Every web browser must provide three basic functions: 1) It must provide
a control interface for human users; 2) it must exchange information with
other computers; and 3) it must interpret HTML and render a web page. We
are primarily interested in this last function—how HTML is understood by
a browser and how that determines what is seen on the page. Many browser
makers use the same open source, HTML rendering engines and difer mostly
in their user interfaces. As a result, only four browser types cover most Web
suring: Internet Explorer, Mozilla (Firefox, Flock), Webkit (Safari, Chrome),
and everything else (mobile phone browsers, legacy versions of IE, and Inter-
net appliances).
As with browsers, several diferent web servers are in use today, hosting
nearly a quarter billion websites in total. By far the most popular web server,
according to a November 2009 survey by Netcrat, is
Apache
, an open-source
product from the Apache Foundation. It hosts about half of all sites worldwide.
he next most popular web server is the Internet Information Server (IIS)
from Microsot, with about one-third of the market. he remaining web serv-
ers are Google Web Server (GWS), which the company uses internally to host
its massive search engine and user sites; nginx (pronounced “engine X”), a
free, lightweight, high-performance server written by Igor Sysoev; and Qzone,
a Chinese web server used by QQ.com to host upward of 20 million blogs
under its domain.
When a web server receives a request from a user agent, all it has to do
is igure out which ile to return. Actually, it is a bit more complicated than
that. Apache, for example, has a modular structure with “hooks” that allow
a systems administrator to include custom components. Apache analyzes
