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NCSA--A Beginner's Guide to HTML, Part 3

Part 3 contains the following sections:

You can also return to Part 1 or return to Part 2.

Inline Images

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.

Image Size Attributes

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.

Aligning Images

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:

<IMG SRC = "BarHotlist.gif"> 
which results in:

The image is centered; this paragraph starts below it and left justified.

Alternate Text for Images

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.

Background Graphics

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"> 

Background Color

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:

External Images, Sounds, and Animations

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="">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.

Table Elements

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.

Table Attributes

NOTE: Attributes defined within <TH> ... </TH> or <TD> ... </TD> cells override the default alignment set in a <TR> ... </TR>.
Attribute Description





  • 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.

General Table Format

The general format of a table looks like this:
  start of table definition 
<CAPTION> caption contents </CAPTION>       
  caption definition 
  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.

Tables for Nontabular Information

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.

Fill-out Forms

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.


Avoid Overlapping Tags

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.

Embed Only Anchors and Character Tags

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> 

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:

    <LI><B>A bold list item</B> 
    <LI><I>An italic list item</I> 

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:

    <LI><H1>A large heading</H1> 
    <LI><H2>Something slightly smaller</H2> 

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.

Do the Final Steps

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.

Commenting Your Files

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.

For More Information

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.

Style Guides

The following offer advice on how to write "good" HTML:

Other Introductory Documents

These cover similar information as this guide:

Additional Online References


NCSA acknowledges and thanks the many Web users who have commented on this guide. Your comments are useful, appreciated, and always welcome. Thanks also to the NCSA reviewers and contributors as well as the author of the first version of this guide.

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A Beginner's Guide to HTML / / revised April 96
edits: 7/96; 9/96; 1/97

The National Center for Supercomputing Applications
University of Illinois at Urbana-Champaign