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- WWW
- World Wide Web
- Web
- World Wide Web
- SGML
- Standard Generalized Markup Language--a standard for describing
markup languages
- DTD
- Document Type Definition--this is the formal specification
of a markup language, written using SGML
- HTML
- HyperText Markup Language--HTML is an SGML DTD
In practical terms, HTML is a collection of platform-independent
styles (indicated by markup tags) that define the various components
of a World Wide Web document. HTML was invented by Tim Berners-Lee
while at CERN, the European Laboratory for Particle Physics in
Geneva.
This primer assumes that you:
- know how to use NCSA Mosaic or some other Web browser
- have a general understanding of how Web servers and client
browsers work
- have access to a Web server (or that you want to produce HTML
documents for personal use in local-viewing mode)
This guide reflects the most current specification--HTML Version
2.0-- plus some additional features that have been widely and consistently
implemented in browsers. Future versions and new features for HTML
are under development.
HTML documents are plain-text (also known as ASCII) files that
can be created using any text editor (e.g., Emacs or vi on UNIX
machines; BBEdit on a Macintosh; Notepad on a Windows machine).
You can also use word-processing software if you remember to save
your document as "text only with line breaks."
Some WYSIWYG editors are available (e.g., HotMetal, which is available
for several platforms or Adobe PageMill for Macintoshes). You may
wish to try one of them after you learn some of the basics of HTML
tagging. It is useful to know enough HTML to code a document before
you determine the usefulness of a WYSIWYG editor.
If you haven't already selected your software, refer to an online
listing of HTML editors (organized by platform) to help you
in your search for appropriate software.
An element is a fundamental component of the structure
of a text document. Some examples of elements are heads, tables,
paragraphs, and lists. Think of it this way: you use HTML tags to
mark the elements of a file for your browser. Elements can contain
plain text, other elements, or both.
To denote the various elements in an HTML document, you use tags.
HTML tags consist of a left angle bracket (<), a
tag name, and a right angle bracket (>). Tags are
usually paired (e.g., <H1> and </H1>)
to start and end the tag instruction. The end tag looks just like
the start tag except a slash (/) precedes the text within the brackets.
HTML tags are listed below.
Some elements may include an attribute, which is additional
information that is included inside the start tag. For example,
you can specify the alignment of images (top, middle, or bottom)
by including the appropriate attribute with the image source HTML
code. Tags that have optional attributes are noted
below.
NOTE: HTML is not case sensitive. <title>
is equivalent to <TITLE> or <TiTlE>.
There are a few exceptions noted in Escape Sequences
below.
Not all tags are supported by all World Wide Web browsers. If
a browser does not support a tag, it (usually) just ignores it.
Every HTML document should contain certain standard HTML tags.
Each document consists of head and body text. The head contains
the title, and the body contains the actual text that is made up
of paragraphs, lists, and other elements. Browsers expect specific
information because they are programmed according to HTML and SGML
specifications.
Required elements are shown in this sample bare-bones document:
<html>
<head>
<TITLE>A Simple HTML Example</TITLE>
</head>
<body>
<H1>HTML is Easy To Learn</H1>
<P>Welcome to the world of HTML.
This is the first paragraph. While short it is
still a paragraph!</P>
<P>And this is the second paragraph.</P>
</body>
</html>
The required elements are the <html>, <head>, <title>,
and <body> tags (and their corresponding end tags). Because
you should include these tags in each file, you might want to create
a template file with them. (Some browsers will format your HTML
file correctly even if these tags are not included. But some browsers
won't! So make sure to include them.)
To see a copy of the file that your browser reads to generate
the information in your current window, select View Source (or the
equivalent) from the browser menu. The file contents, with all the
HTML tags, are displayed in a new window.
This is an excellent way to see how HTML is used and to learn
tips and constructs. Of course, the HTML might not be technically
correct. Once you become familiar with HTML and check the many online
and hard-copy references on the subject, you will learn to distinguish
between "good" and "bad" HTML.
Remember that you can save a source file with the HTML codes and
use it as a template for one of your Web pages or modify the format
to suit your purposes.
This element tells your browser that the file contains HTML-coded
information. The file extension .html also indicates
this an HTML document and must be used. (If you are restricted to
8.3 filenames (e.g., LeeHome.htm, use only .htm
for your extension.)
The head element identifies the first part of your HTML-coded
document that contains the title. The title is shown as part of
your browser's window (see below).
The title element contains your document title and identifies
its content in a global context. The title is displayed somewhere
on the browser window (usually at the top), but not within the text
area. The title is also what is displayed on someone's hotlist or
bookmark list, so choose something descriptive, unique, and relatively
short. A title is also used during a WAIS search of a server.
For example, you might include a shortened title of a book along
with the chapter contents: NCSA Mosaic Guide (Windows): Installation.
This tells the software name, the platform, and the chapter contents,
which is more useful than simply calling the document Installation.
Generally you should keep your titles to 64 characters or fewer.
The second--and largest--part of your HTML document is the body,
which contains the content of your document (displayed within the
text area of your browser window). The tags explained below are
used within the body of your HTML document.
HTML has six levels of headings, numbered 1 through 6, with 1
being the most prominent. Headings are displayed in larger and/or
bolder fonts than normal body text. The first heading in each document
should be tagged <H1>.
The syntax of the heading element is:
<Hy>Text of heading
</Hy>
where y is a number between 1 and 6 specifying the level
of the heading.
Do not skip levels of headings in your document. For example,
don't start with a level-one heading (<H1>) and then next
use a level-three (<H3>) heading.
Unlike documents in most word processors, carriage returns in
HTML files aren't significant. So you don't have to worry about
how long your lines of text are (better to have them fewer than
72 characters long though). Word wrapping can occur at any point
in your source file, and multiple spaces are collapsed into a single
space by your browser.
In the bare-bones example shown in the Minimal HTML Document section,
the first paragraph is coded as
<P>Welcome to the world of HTML.
This is the first paragraph.
While short it is
still a paragraph!</P>
In the source file there is a line break between the sentences.
A Web browser ignores this line break and starts a new paragraph
only when it encounters another <P> tag.
Important: You must indicate paragraphs with
<P> elements. A browser ignores any indentations or blank
lines in the source text. Without <P> elements, the document
becomes one large paragraph. (One exception is text tagged as "preformatted,"
which is explained below.) For example, the following would produce
identical output as the first bare-bones HTML example:
<H1>Level-one heading</H1>
<P>Welcome to the world of HTML. This is the
first paragraph. While short it is still a
paragraph! </P> <P>And this is the second paragraph.</P>
To preserve readability in HTML files, put headings on separate
lines, use a blank line or two where it helps identify the start
of a new section, and separate paragraphs with blank lines (in addition
to the <P> tags). These extra spaces will help
you when you edit your files (but your browser will ignore the extra
spaces because it has its own set of rules on spacing that do not
depend on the spaces you put in your source file).
NOTE: The </P> closing tag
can be omitted. This is because browsers understand that when they
encounter a <P> tag, it implies that there is an end to the
previous paragraph.
Using the <P> and </P> as a paragraph container means
that you can center a paragraph by including the ALIGN=alignment
attribute in your source file.
<P ALIGN=CENTER>
This is a centered paragraph.
[See the formatted version below.]
</P>
This is a centered paragraph.
HTML supports unnumbered, numbered, and definition lists. You
can nest lists too, but use this feature sparingly because too many
nested items can get difficult to follow.
Unnumbered Lists
To make an unnumbered, bulleted list,
- start with an opening list <UL> (for unnumbered
list) tag
- enter the <LI> (list item) tag followed
by the individual item; no closing </LI> tag
is needed
- end the entire list with a closing list </UL>
tag
Below is a sample three-item list:
<UL>
<LI> apples
<LI> bananas
<LI> grapefruit
</UL>
The output is:
- apples
- bananas
- grapefruit
The <LI> items can contain multiple paragraphs.
Indicate the paragraphs with the <P> paragraph
tags.
Numbered Lists
A numbered list (also called an ordered list, from
which the tag name derives) is identical to an unnumbered list,
except it uses <OL> instead of <UL>.
The items are tagged using the same <LI> tag.
The following HTML code:
<OL>
<LI> oranges
<LI> peaches
<LI> grapes
</OL>
produces this formatted output:
- oranges
- peaches
- grapes
Definition Lists
A definition list (coded as <DL>) usually consists
of alternating a definition term (coded as <DT>)
and a definition definition (coded as <DD>).
Web browsers generally format the definition on a new line.
The following is an example of a definition list:
<DL>
<DT> NCSA
<DD> NCSA, the National Center for Supercomputing
Applications, is located on the campus of the
University of Illinois at Urbana-Champaign.
<DT> Cornell Theory Center
<DD> CTC is located on the campus of Cornell
University in Ithaca, New York.
</DL>
The output looks like:
- NCSA
- NCSA, the National Center for Supercomputing Applications,
is located on the campus of the University of Illinois at Urbana-Champaign.
- Cornell Theory Center
- CTC is located on the campus of Cornell University in Ithaca,
New York.
The <DT> and <DD> entries
can contain multiple paragraphs (indicated by <P>
paragraph tags), lists, or other definition information.
The COMPACT attribute can be used routinely in case
your definition terms are very short. If, for example, you are showing
some computer options, the options may fit on the same line as the
start of the definition.
<DL COMPACT>
<DT> -i
<DD>invokes NCSA Mosaic for Microsoft Windows
using the initialization file defined in the path
<DT> -k
<DD>invokes NCSA Mosaic for Microsoft Windows in
kiosk mode
</DL>
The output looks like:
- -i
- invokes NCSA Mosaic for Microsoft Windows using the initialization
file defined in the path.
- -k
- invokes NCSA Mosaic for Microsoft Windows in kiosk mode.
Nested Lists
Lists can be nested. You can also have a number of paragraphs,
each containing a nested list, in a single list item.
Here is a sample nested list:
<UL>
<LI> A few New England states:
<UL>
<LI> Vermont
<LI> New Hampshire
<LI> Maine
</UL>
<LI> Two Midwestern states:
<UL>
<LI> Michigan
<LI> Indiana
</UL>
</UL>
The nested list is displayed as
- A few New England states:
- Vermont
- New Hampshire
- Maine
- Two Midwestern states:
Use the <PRE> tag (which stands for "preformatted")
to generate text in a fixed-width font. This tag also makes spaces,
new lines, and tabs significant (multiple spaces are displayed as
multiple spaces, and lines break in the same locations as in the
source HTML file). This is useful for program listings, among other
things. For example, the following lines:
<PRE>
#!/bin/csh
cd $SCR
cfs get mysrc.f:mycfsdir/mysrc.f
cfs get myinfile:mycfsdir/myinfile
fc -02 -o mya.out mysrc.f
mya.out
cfs save myoutfile:mycfsdir/myoutfile
rm *
</PRE>
display as:
#!/bin/csh
cd $SCR
cfs get mysrc.f:mycfsdir/mysrc.f
cfs get myinfile:mycfsdir/myinfile
fc -02 -o mya.out mysrc.f
mya.out
cfs save myoutfile:mycfsdir/myoutfile
rm *
The <PRE> tag can be used with an optional
WIDTH attribute that specifies the maximum number of
characters for a line. WIDTH also signals your browser
to choose an appropriate font and indentation for the text.
Hyperlinks can be used within <PRE> sections.
You should avoid using other HTML tags within <PRE>
sections, however.
Note that because <, >, and & have special meanings
in HTML, you must use their escape sequences (<,
>, and &, respectively)
to enter these characters. See the section Escape
Sequences for more information.
Use the <BLOCKQUOTE> tag to include lengthy
quotations in a separate block on the screen. Most browsers generally
change the margins for the quotation to separate it from surrounding
text.
In the example:
<P>Omit needless words.</P>
<BLOCKQUOTE>
<P>Vigorous writing is concise. A sentence should
contain no unnecessary words, a paragraph no unnecessary
sentences, for the same reason that a drawing should have
no unnecessary lines and a machine no unnecessary parts.
</P>
<P>--William Strunk, Jr., 1918 </P>
</BLOCKQUOTE>
the result is:
Omit needless words.
Vigorous writing is concise. A sentence should contain
no unnecessary words, a paragraph no unnecessary sentences, for
the same reason that a drawing should have no unnecessary lines
and a machine no unnecessary parts.
--William Strunk, Jr., 1918
The <ADDRESS> tag is generally used to specify
the author of a document, a way to contact the author (e.g., an
email address), and a revision date. It is usually the last item
in a file.
For example, the last line of the online version of this guide
is:
<ADDRESS>
A Beginner's Guide to HTML / NCSA /
pubs@ncsa.uiuc.edu / revised April 96
</ADDRESS>
The result is:
A Beginner's Guide to HTML / NCSA / pubs@ncsa.uiuc.edu /
revised April 96
NOTE: <ADDRESS> is not
used for postal addresses. See "Forced Line Breaks" below to see
how to format postal addresses.
The <BR> tag forces a line break with no extra
(white) space between lines. Using <P> elements
for short lines of text such as postal addresses results in unwanted
additional white space. For example, with <BR>:
National Center for Supercomputing Applications<BR>
605 East Springfield Avenue<BR>
Champaign, Illinois 61820-5518<BR>
The output is:
National Center for Supercomputing Applications
605 East Springfield Avenue
Champaign, Illinois 61820-5518
The <HR> tag produces a horizontal line the
width of the browser window. A horizontal rule is useful to separate
sections of your document. For example, many people add a rule at
the end of their text and before the <address>
information.
You can vary a rule's size (thickness) and width (the percentage
of the window covered by the rule). Experiment with the settings
until you are satisfied with the presentation. For example:
<HR SIZE=4 WIDTH="50%">
displays as:
HTML has two types of styles for individual words or sentences:
logical and physical. Logical styles tag text according
to its meaning, while physical styles indicate the specific
appearance of a section. For example, in the preceding sentence,
the words "logical styles" was tagged as a "definition." The same
effect (formatting those words in italics) could have been achieved
via a different tag that tells your browser to "put these words
in italics."
NOTE: Some browsers don't attach any style to
the <DFN> tag, so you might not see the indicated
phrases in the previous paragraph in italics.
If physical and logical styles produce the same result on the
screen, why are there both?
In the ideal SGML universe, content is divorced from presentation.
Thus SGML tags a level-one heading as a level-one heading, but does
not specify that the level-one heading should be, for instance,
24-point bold Times centered. The advantage of this approach (it's
similar in concept to style sheets in many word processors) is that
if you decide to change level-one headings to be 20-point left-justified
Helvetica, all you have to do is change the definition of the level-one
heading in your Web browser. Indeed many browsers today let you
define how you want the various HTML tags rendered on-screen.
Another advantage of logical tags is that they help enforce consistency
in your documents. It's easier to tag something as <H1>
than to remember that level-one headings are 24-point bold Times
centered or whatever. For example, consider the <STRONG>
tag. Most browsers render it in bold text. However, it is possible
that a reader would prefer that these sections be displayed in red
instead. Logical styles offer this flexibility.
Of course, if you want something to be displayed in italics (for
example) and do not want a browser's setting to display it differently,
use physical styles. Physical styles, therefore, offer consistency
in that something you tag a certain way will always be displayed
that way for readers of your document.
Try to be consistent about which type of style you use. If you
tag with physical styles, do so throughout a document. If you use
logical styles, stick with them within a document. Keep in mind
that future releases of HTML might not support physical styles,
which could mean that browsers will not display physical style coding.
Logical Styles
- <DFN>
- for a word being defined. Typically displayed in italics. (NCSA
Mosaic is a World Wide Web browser.)
- <EM>
- for emphasis. Typically displayed in italics. (Consultants
cannot reset your password unless you call the help line.)
- <CITE>
- for titles of books, films, etc. Typically displayed in italics.
(A Beginner's Guide to HTML)
- <CODE>
- for computer code. Displayed in a fixed-width font. (The <stdio.h>
header file)
- <KBD>
- for user keyboard entry. Typically displayed in plain fixed-width
font. (Enter passwd to change your password.)
- <SAMP>
- for a sequence of literal characters. Displayed in a fixed-width
font. (Segmentation fault: Core dumped.)
- <STRONG>
- for strong emphasis. Typically displayed in bold. (NOTE:
Always check your links.)
- <VAR>
- for a variable, where you will replace the variable with specific
information. Typically displayed in italics. (rm filename
deletes the file.)
Physical Styles
- <B>
- bold text
- <I>
- italic text
- <TT>
- typewriter text, e.g. fixed-width font.
Character entities have two functions:
- escaping special characters
- displaying other characters not available in the plain ASCII
character set (primarily characters with diacritical marks)
Three ASCII characters--the left angle bracket (<), the right
angle bracket (>), and the ampersand (&)--have special meanings
in HTML and therefore cannot be used "as is" in text. (The angle
brackets are used to indicate the beginning and end of HTML tags,
and the ampersand is used to indicate the beginning of an escape
sequence.) Double quote marks may be used as-is but a character
entity may also be used (").
To use one of the three characters in an HTML document, you must
enter its escape sequence instead:
- <
- the escape sequence for <
- >
- the escape sequence for >
- &
- the escape sequence for &
Additional escape sequences support accented characters, such
as:
- ö
- the escape sequence for a lowercase o with an umlaut: ö
- ñ
- the escape sequence for a lowercase n with an tilde: ñ
- È
- the escape sequence for an uppercase E with a grave accent:
È
You can substitute other letters for the o, n, and
E shown above. Check this online reference for a longer list
of
special characters.
NOTE: Unlike the rest of HTML, the escape sequences
are case sensitive. You cannot, for instance, use <
instead of <.
The chief power of HTML comes from its ability to link text and/or
an image to another document or section of a document. A browser
highlights the identified text or image with color and/or underlines
to indicate that it is a hypertext link (often shortened
to hyperlink or link).
HTML's single hypertext-related tag is <A>,
which stands for anchor. To include an anchor in your
document:
- start the anchor with <A (include a space after
the
A)
- specify the document you're linking to by entering the parameter
HREF="filename" followed
by a closing right angle bracket (>)
- enter the text that will serve as the hypertext link in the
current document
- enter the ending anchor tag: </A> (no space
is needed before the end anchor tag)
Here is a sample hypertext reference in a file called US.html:
<A HREF="MaineStats.html">Maine</A>
This entry makes the word Maine the hyperlink to the document
MaineStats.html, which is in the same directory as
the first document.
You can link to documents in other directories by specifying the
relative path from the current document to the linked
document. For example, a link to a file NYStats.html
located in the subdirectory AtlanticStates would be:
<A HREF="AtlanticStates/NYStats.html">New York</A>
These are called relative links because you are specifying
the path to the linked file relative to the location of the current
file. You can also use the absolute pathname (the complete URL)
of the file, but relative links are more efficient in accessing
a server.
Pathnames use the standard UNIX syntax. The UNIX syntax for the
parent directory (the directory that contains the current directory)
is "..". (For more information consult a beginning UNIX reference
text such as Learning the UNIX Operating System from
O'Reilly and Associates, Inc.)
If you were in the NYStats.html file and were referring
to the original document US.html, your link would look
like this:
<A HREF="../US.html">United States</A>
In general, you should use relative links because:
- it's easier to move a group of documents to another location
(because the relative path names will still be valid)
- it's more efficient connecting to the server
- there is less to type
However use absolute pathnames when linking to documents that
are not directly related. For example, consider a group of documents
that comprise a user manual. Links within this group should be relative
links. Links to other documents (perhaps a reference to related
software) should use full path names. This way if you move the user
manual to a different directory, none of the links would have to
be updated.
The World Wide Web uses Uniform Resource Locators (URLs) to specify
the location of files on other servers. A URL includes the type
of resource being accessed (e.g., Web, gopher, WAIS), the address
of the server, and the location of the file. The syntax is:
scheme://host.domain
[:port]/path/
filename
where scheme is one of
- file
- a file on your local system
- ftp
- a file on an anonymous FTP server
- http
- a file on a World Wide Web server
- gopher
- a file on a Gopher server
- WAIS
- a file on a WAIS server
- news
- a Usenet newsgroup
- telnet
- a connection to a Telnet-based service
The port number can generally
be omitted. (That means unless someone tells you otherwise, leave
it out.)
For example, to include a link to this primer in your document,
enter:
<A HREF="http://www.ncsa.uiuc.edu/General/Internet/WWW/HTMLPrimer.html">
NCSA's Beginner's Guide to HTML</A>
This entry makes the text NCSA's Beginner's Guide to HTML
a hyperlink to this document.
For more information on URLs, refer to:
Anchors can also be used to move a reader to a particular section
in a document (either the same or a different document) rather than
to the top, which is the default. This type of an anchor is commonly
called a named anchor because to create the links, you
insert HTML names within the document.
This guide is a good example of using named anchors in one document.
The guide is constructed as one document to make printing easier.
But as one (long) document, it can be time-consuming to move through
when all you really want to know about is one bit of information
about HTML. Internal hyperlinks are used to create a "table of contents"
at the top of this document. These hyperlinks move you from one
location in the document to another location in the same document.
(Go to the top of this document and then click
on the Links to Specific Sections hyperlink in the
table of contents. You will wind up back here.)
You can also link to a specific section in another document. That
information is presented first because understanding that helps
you understand linking within one document.
Links Between Sections of Different Documents
Suppose you want to set a link from document A (documentA.html)
to a specific section in another document (MaineStats.html).
Enter the HTML coding for a link to a named anchor:
documentA.html:
In addition to the many state parks, Maine is also home to
<a href="MaineStats.html#ANP">Acadia National Park</a>.
Think of the characters after the hash (#) mark as a tab within
the MaineStats.html file. This tab tells your browser
what should be displayed at the top of the window when the link
is activated. In other words, the first line in your browser window
should be the Acadia National Park heading.
Next, create the named anchor (in this example "ANP")
in MaineStats.html:
<H2><A NAME="ANP">Acadia National Park</a></H2>
With both of these elements in place, you can bring a reader directly
to the Acadia reference in MaineStats.html.
NOTE: You cannot make links to specific sections
within a different document unless either you have write permission
to the coded source of that document or that document already contains
in-document named anchors. For example, you could include named
anchors to this primer in a document you are writing because there
are named anchors in this guide (use View Source in your browser
to see the coding). But if this document did not have named
anchors, you could not make a link to a specific section because
you cannot edit the original file on NCSA's server.
Links to Specific Sections within the Current Document
The technique is the same except the filename is omitted.
For example, to link to the ANP anchor from within
MaineStats, enter:
...More information about
<A HREF="#ANP">Acadia National Park</a>
is available elsewhere in this document.
Be sure to include the <A NAME=> tag at the
place in your document where you want the link to jump to (<H2><A
NAME="ANP">Acadia National Park</a></H2>).
Named anchors are particularly useful when you think readers will
print a document in its entirety or when you have a lot of short
information you want to place online in one file.
You can make it easy for a reader to send electronic mail to a
specific person or mail alias by including the mailto
attribute in a hyperlink. The format is:
<A HREF="mailto:emailinfo@host">Name</a>
For example, enter:
<A HREF="mailto:pubs@ncsa.uiuc.edu">
NCSA Publications Group</a>
to create a mail window that is already configured to open a mail
window for the NCSA Publications
Group alias. (You, of course, will enter another mail address!)
Most Web browsers can display inline images (that is, images next
to text) that are in X Bitmap (XBM), GIF, or JPEG format. Other
image formats are being incorporated into Web browsers [e.g., the
Portable Network Graphic (PNG) format]. Each image takes time to
process and slows down the initial display of a document. Carefully
select your images and the number of images in a document.
To include an inline image, enter:
<IMG SRC=ImageName>
where ImageName is the URL of the image file.
The syntax for <IMG SRC> URLs is identical
to that used in an anchor HREF. If the image file is
a GIF file, then the filename part of ImageName must
end with .gif. Filenames of X Bitmap images must end
with .xbm; JPEG image files must end with .jpg
or .jpeg; and Portable Network Graphic files must end
with .png.
You should include two other attributes on <IMG>
tags to tell your browser the size of the images it is downloading
with the text. The HEIGHT and WIDTH attributes
let your browser set aside the appropriate space (in pixels) for
the images as it downloads the rest of the file. (Get the pixel
size from your image-processing software, such as Adobe Photoshop.)
For example, to include a self portrait image in a file along
with the portrait's dimensions, enter:
<IMG SRC=SelfPortrait.gif HEIGHT=100 WIDTH=65>
NOTE: Some browsers use the HEIGHT
and WIDTH attributes to stretch or shrink an image
to fit into the allotted space when the image does not exactly match
the attribute numbers. Not all browser developers think stretching/shrinking
is a good idea. So don't plan on your readers having access to this
feature. Check your dimensions and use the correct ones.
You have some flexibility when displaying images. You can have
images separated from text and aligned to the left or right or centered.
Or you can have an image aligned with text. Try several possibilities
to see how your information looks best.
Aligning Text with an Image
 By default the bottom of an
image is aligned with the following text, as shown in this paragraph.
You can align images to the top or center of a paragraph using the
ALIGN= attributes TOP and CENTER.
This text is aligned
with the top of the image (<IMG SRC = "BarHotlist.gif"
ALIGN=TOP>). Notice how the browser aligns only one line
and then jumps to the bottom of the image for the rest of the text.
And this text
is centered on the image (<IMG SRC = "BarHotlist.gif" ALIGN=CENTER>).
Again, only one line of text is centered; the rest is below the
image.
Images without Text
To display an image without any associated text (e.g., your organization's
logo), make it a separate paragraph. Use the paragraph ALIGN=
attribute to center the image or adjust it to the right side of
the window as shown below:
<p ALIGN=CENTER>
<IMG SRC = "BarHotlist.gif">
</p>
which results in:
The image is centered; this paragraph starts below it and left
justified.
Some World Wide Web browsers--primarily those that run on VT100
terminals--cannot display images. Some users turn off image loading
even if their software can display images (especially if they are
using a modem or have a slow connection). HTML provides a mechanism
to tell readers what they are missing on your pages.
The ALT attribute lets you specify text to be displayed
instead of an image. For example:
<IMG SRC="UpArrow.gif" ALT="Up">
where UpArrow.gif is the picture of an upward pointing
arrow. With graphics-capable viewers that have image-loading turned
on, you see the up arrow graphic. With a VT100 browser or if image-loading
is turned off, the word Up is shown in your window.
You should try to include alternate text for each image you use
in your document, which is a courtesy for your readers.
Newer versions of Web browsers can load an image and use it as
a background when displaying a page. Some people like background
images and some don't. In general, if you want to include a background,
make sure your text can be read easily when displayed on top of
the image.
Background images can be a texture (linen finished paper, for
example) or an image of an object (a logo possibly). You create
the background image as you do any image.
However you only have to create a small piece of the image. Using
a feature called tiling, a browser takes the image and repeats it
across and down to fill your browser window. In sum you generate
one image, and the browser replicates it enough times to fill your
window. This action is automatic when you use the background tag
shown below.
The tag to include a background image is included in the <BODY>
statement as an attribute:
<BODY BACKGROUND="filename.gif">
By default browsers display text in black on a gray background.
However, you can change both elements if you want. Some HTML authors
select a background color and coordinate it with a change in the
color of the text.
Always preview changes like this to make sure your pages are readable.
(For example, many people find red text on a black background difficult
to read!)
You change the color of text, links, visited links, and active
links using attributes of the <BODY> tag. For
example, enter:
<BODY BGCOLOR="#000000" TEXT="#FFFFFF" LINK="#9690CC">
This creates a window with a black background (BGCOLOR),
white text (TEXT), and silvery hyperlinks (LINK).
The six-digit number and letter combinations represent colors
by giving their RGB (red, green, blue) value. The six digits are
actually three two-digit numbers in sequence, representing the amount
of red, green, or blue as a hexadecimal value in the range 00-FF.
For example, 000000 is black (no color at all), FF0000 is bright
red, and FFFFFF is white (fully saturated with all three colors).
These number and letter combinations are cryptic. Fortunately an
online resource is available to help you track down the combinations
that map to specific colors and there is software available for
you to do this on your workstation:
You may want to have an image open as a separate document when
a user activates a link on either a word or a smaller, inline version
of the image included in your document. This is called an external
image, and it is useful if you do not wish to slow down the loading
of the main document with large inline images.
To include a reference to an external image, enter:
<A HREF="MyImage.gif">link anchor</A>
You can also use a smaller image as a link to a larger image.
Enter:
<A HREF="LargerImage.gif"><IMG SRC="SmallImage.gif"></A>
The reader sees the SmallImage.gif image and clicks
on it to open the LargerImage.gif file.
Use the same syntax for links to external animations and sounds.
The only difference is the file extension of the linked file. For
example,
<A HREF="AdamsRib.mov">link
anchor</A>
specifies a link to a QuickTime movie. Some common file types
and their extensions are:
| File Type |
Extension |
| plain text |
.txt |
| HTML document |
.html |
| GIF image |
.gif |
| TIFF image |
.tiff |
| X Bitmap image |
.xbm |
| JPEG image |
.jpg or .jpeg |
| PostScript file |
.ps |
| AIFF sound file |
.aiff |
| AU sound file |
.au |
| WAV sound file |
.wav |
| QuickTime movie |
.mov |
| MPEG movie |
.mpeg or .mpg |
Keep in mind your intended audience and their access to software.
Most UNIX workstations, for instance, cannot view QuickTime movies.
Before HTML tags for tables were finalized, authors had to carefully
format their tabular information within <PRE>
tags, counting spaces and previewing their output. Tables are very
useful for presentation of tabular information as well as a boon
to creative HTML authors who use the table tags to present their
regular Web pages. (Check out the NCSA
Relativity Group's pages for an excellent, award-winning example.)
Think of your tabular information in light of the coding explained
below. A table has heads where you explain what the columns/rows
include, rows for information, cells for each item. In the following
table, the first column contains the header information, each row
explains an HTML table tag, and each cell contains a paired tag
or an explanation of the tag's function.
|
|
| Element |
Description |
| <TABLE> ... </TABLE> |
defines a table in HTML. If the BORDER attribute
is present, your browser displays the table with a border. |
| <CAPTION> ... </CAPTION> |
defines the caption for the title of the table. The default
position of the title is centered at the top of the table. The
attribute ALIGN=BOTTOM can be used to position
the caption below the table.
NOTE: Any kind of markup tag can be used in the caption. |
| <TR> ... </TR> |
specifies a table row within a table. You may define default
attributes for the entire row: ALIGN (LEFT,
CENTER, RIGHT) and/or VALIGN
(TOP, MIDDLE, BOTTOM).
See Table Attributes at the end of this table for more information. |
| <TH> ... </TH> |
defines a table header cell. By default the text in this cell
is bold and centered. Table header cells may contain other attributes
to determine the characteristics of the cell and/or its contents.
See Table Attributes at the end of this table for more information. |
| <TD> ... </TD> |
defines a table data cell. By default the text in this cell
is aligned left and centered vertically. Table data cells may
contain other attributes to determine the characteristics of
the cell and/or its contents. See Table Attributes at the end
of this table for more information. |
|
|
| NOTE: Attributes defined within
<TH> ... </TH> or <TD>
... </TD> cells override the default alignment
set in a <TR> ... </TR>. |
| Attribute |
Description |
- ALIGN (LEFT, CENTER, RIGHT)
- VALIGN (TOP, MIDDLE, BOTTOM)
- COLSPAN=n
- ROWSPAN=n
- NOWRAP
|
- Horizontal alignment of a cell.
- Vertical alignment of a cell.
- The number (n) of columns a cell spans.
- The number (n) of rows a cell spans.
- Turn off word wrapping within a cell.
|
The general format of a table looks like this:
<TABLE>
start of table definition
<CAPTION> caption contents </CAPTION>
caption definition
<TR>
start of first row definition
<TH> cell contents </TH>
first cell in row 1 (a head)
<TH> cell contents </TH>
last cell in row 1 (a head)
</TR>
end of first row definition
<TR>
start of second row definition
<TD> cell contents </TD>
first cell in
row 2
<TD> cell contents </TD>
last cell in row 2
</TR>
end of second row definition
<TR>
start of last row definition
<TD> cell contents </TD>
first cell in last row ...
<TD> cell contents </TD>
last cell in last row
</TR>
end of last row definition
</TABLE>
end of table definition
The <TABLE> and </TABLE>
tags must surround the entire table definition. The first
item inside the table is the CAPTION, which is optional.
Then you can have any number of rows defined by the <TR>
and </TR> tags. Within a row you can have any
number of cells defined by the <TD>...</TD>
or <TH>...</TH> tags. Each
row of a table is, essentially, formatted independently of the rows
above and below it. This lets you easily display tables like the
one above with a single cell, such as Table Attributes, spanning
columns of the table.
Some HTML authors use tables to present nontabular information.
For example, because links can be included in table cells, some
authors use a table with no borders to create "one" image from separate
images. Browsers that can display tables properly show the various
images seamlessly, making the created image seem like an image
map (one image with hyperlinked quadrants).
Using table borders with images can create an impressive display
as well. Experiment and see what you like.
Web forms let a reader return information to a Web server for
some action. For example, suppose you collect names and email addresses
so you can email some information to people who request it. For
each person who enters his or her name and address, you need some
information to be sent and the respondent's particulars added to
a data base.
This processing of incoming data is usually handled by a script
or program written in Perl or another language that manipulates
text, files, and information. If you cannot write a program or script
for your incoming information, you need to find someone who can
do this for you.
The forms themselves are not hard to code. They follow the same
constructs as other HTML tags. What could be difficult is the program
or script that takes the information submitted in a form and processes
it. Because of the need for specialized scripts to handle the incoming
form information, fill-out
forms are not discussed in this primer. Check the Additional
Online Reference section for more information.
Consider this example of HTML:
<B>This is an example of <DFN>overlapping</B>
HTML tags.</DFN>
The word overlapping is contained within both the <B>
and <DFN> tags. A browser might be confused by
this coding and might not display it the way you intend. The only
way to know is to check each popular browser (which is time-consuming
and impractical).
In general, avoid overlapping tags. Look at your tags and try
pairing them up. Tags (with the obvious exceptions of elements whose
end tags may be omitted, such as paragraphs) should be paired without
an intervening tag in between. Look again at the example above.
You cannot pair the bold tags without another tag in the middle
(the first definition tag). Try matching your coding up like this
to see if you have any problem areas that should be fixed before
your release your files to a server.
HTML protocol allows you to embed links within other HTML tags:
<H1><A HREF="Destination.html">My heading</A></H1>
Do not embed HTML tags within an anchor:
<A HREF="Destination.html">
<H1>My heading</H1>
</A>
Although most browsers currently handle this second example, the
official HTML specifications do not support this construct and your
file will probably not work with future browsers. Remember that
browsers can be forgiving when displaying improperly coded files.
But that forgiveness may not last to the next version of the software!
When in doubt, code your files according to the HTML specifications
(see For More Information below).
Character tags modify the appearance of the text within other
elements:
<UL>
<LI><B>A bold list item</B>
<LI><I>An italic list item</I>
</UL>
Avoid embedding other types of HTML element tags. For example,
you might be tempted to embed a heading within a list in order to
make the font size larger:
<UL>
<LI><H1>A large heading</H1>
<LI><H2>Something slightly smaller</H2>
</UL>
Although some browsers handle this quite nicely, formatting of
such coding is unpredictable (because it is undefined). For compatibility
with all browsers, avoid these kinds of constructs. (The Netscape
<FONT> tag, which lets you specify how large individual characters
will be displayed in your window, is not currently part of the official
HTML specifications.)
What's the difference between embedding a <B>
within a <LI> tag as opposed to embedding a <H1>
within a <LI>? Within HTML the semantic meaning
of <H1> is that it's the main heading of a document
and that it should be followed by the content of the document. Therefore
it doesn't make sense to find a <H1> within a
list.
Character formatting tags also are generally not additive. For
example, you might expect that:
<B><I>some text</I></B>
would produce bold-italic text. On some browsers it does; other
browsers interpret only the innermost tag.
Validate Your Code
When you put a document on a Web server, be sure to check the
formatting and each link (including named anchors). Ideally you
will have someone else read through and comment on your file(s)
before you consider a document finished.
You can run your coded files through
an HTML validation service that will tell you if your code conforms
to accepted HTML. If you are not sure your coding conforms to HTML
specifications, this can be a useful teaching tool. Fortunately
the service lets you select the level of conformance you want for
your files (i.e., strict, level 2, level 3). If you want to use
some codes that are not officially part of the HTML specifications,
this latitude is helpful.
Dummy Images
When an <IMG SRC> tag points to an image that
does not exist, a dummy image is substituted by your browser software.
When this happens during your final review of your files, make sure
that the referenced image does in fact exist, that the hyperlink
has the correct information in the URL, and that the file permission
is set appropriately (world-readable). Then check online again!
Update Your Files
If the contents of a file are static (such as a biography of George
Washington), no updating is probably needed. But for documents that
are time sensitive or covering a field that changes frequently,
remember to update your documents!
Updating is particularly important when the file contains information
such as a weekly schedule or a deadline for a program funding announcement.
Remove out-of-date files or note why something that appears dated
is still on a server (e.g., the program requirements will remain
the same for the next cycle so the file is still available as an
interim reference).
Browsers Differ
Web browsers display HTML elements differently. Remember that
not all codes used in HTML files are interpreted by all browsers.
Any code a browser does not understand is usually ignored though.
You could spend a lot of time making your file "look perfect"
using your current browser. If you check that file using another
browser, it will likely display (a little or a lot) differently.
Hence these words of advice: code your files using correct HTML.
Leave the interpreting to the browsers and hope for the best.
You might want to include comments in your HTML files. Comments
in HTML are like comments in a computer program--the text you enter
is not used by the browser in any formatting and is not directly
viewable by the reader just as computer program comments are not
used and are not viewable. The comments are accessible if a reader
views the source file, however.
Comments such as the name of the person updating a file, the software
and version used in creating a file, or the date that a minor edit
was made are the norm.
To include a comment, enter:
<!-- your comments here -->
You must include the exclamation mark and the hyphens as shown.
This guide is only an introduction to HTML, not
a comprehensive reference. Below are additional online sources of
information. Remember to check a bookstore near you for Web and
HTML books.
The following offer advice on how to write "good" HTML:
These cover similar information as this guide:
This document courtesy of the National Center
for Supercomputing Applications
University of Illinois at Urbana-Champaign
|
Backgrounds Sampler
