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Surely, if you have decided to learn about
XML,
you are probably already quite familiar with the concepts
behind
HTML
(HyperText Markup Language). So let's start
from there.
HTML, as its name implies, is a markup language. As such,
it is used to markup text. But what exactly does it mean
to markup text?
Abstractly, marking up text is a methodology for
encoding data with information about itself. Examples of
markups (encoded data) are ubiquitous in the real world.
For example, back when you were slogging through high school,
you probably used to use a bright yellow highlighter pen to
highlight sentences in your schoolbooks (or at last you knew
someone who did!). You did so because you thought that the
highlighted sentences would be useful to review around exam
time and you wanted a quick way to skim through the important
points. Just like you, thousands of kids around the world did
the exact same thing for the exact same reason.
By highlighting certain bits of text, you were effectively
"marking-up" the data. Essentially, you specified that certain
sentences (data) were important by marking them in yellow.
These sentences became encoded with the fact that they were
important.
And what's more, since everyone followed the same
standard of marking up, you could easily pick up a used text
book and get a good idea just from reading the highlighted
sections what were core points of the book.
There are two crucial points to take away from this example. For
markups to transmit useful information about data to a pool of
users...
- a standard must be in place to define what a valid markup is -
In the example above, markup is defined as a bit of yellow
ink atop text. In HTML a markup is a
tag.
- a standard must be in place to define what markup means -
In the example above, a yellow highlight means the highlighted
text represents an important point. In HTML each tag communicates
its own layout of formatting meaning.
Markups are also ubiquitous in the world of computers. They
are used by word processors to specify formatting and layout,
by communications programs to express the meaning of data sent
over the wires, by database applications that must associate
meaning and relationships with the data they serve, and by
multimedia processing programs which must express meta-data
about images or sound.
As data is sent through dumb computers and programs, it is
essential that the data carries with it information necessary
to communicate what the data means and/or what the receiver
should do with that data.
Data with no context is meaningless just as an unhighlighted
book is bad news around exam time!
HTML
is one of the more famous computer markup systems.
HTML defines a set of
tags
that associate formatting rules
with bits of text. Documents which have been marked up
(which contain plain text as well as the tags that specify
the rules for formatting that text) are read by an HTML
processing application (a web browser for example) that
knows how to display the text according to the rules.
For example, the <B> tag specifies a rule which instructs
an HTML processing application to bold a specific bit of text.
Similarly, the <CENTER> tag instructs the HTML
processing application to center the text.
Thus <CENTER><B>BOLD</B></CENTER>
would be displayed by an HTML processing application as
BOLD
You might imagine a client contact list which could look like the
following bit of HTML code:
<UL>
<LI>Gunther Birznieks
<UL>
<LI>Client ID: 001
<LI>Company: Bob's Fish Store
<LI>Email: gunther@bobsfishstore.com
<LI>Phone: 662-9999
<LI>Street Address: 1234 4th St.
<LI>City: New York
<LI>State: New York
<LI>Zip: 10024
</UL>
<LI>Susan Czigany
<UL>
<LI>Client ID: 002
<LI>Company: Netscape
<LI>Email: susan@eudora.org
<LI>Phone: 555-1234
<LI>Street Address: 9876 Hazen Blvd.
<LI>City: San Jose
<LI>State: California
<LI>Zip: 90034
</UL>
</UL>
The above HTML-encoded data would be displayed by an HTML
processing application as:
- Gunther Birznieks
- Client ID: 001
- Company: Bob's Fish Store
- Email: gunther@bobsfishstore.com
- Phone: 662-9999
- Street Address: 1234 4th St.
- City: New York
- State: New York
- Zip: 10024
- Susan Czigany
- Client ID: 002
- Company: Netscape
- Email: susan@eudora.org
- Phone: 555-1234
- Street Address: 9876 Hazen Blvd.
- City: San Jose
- State: California
- Zip: 90034
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