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Linking

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:

1. start the anchor with <A (include a space after the A)
2. specify the document you're linking to by entering the parameter HREF="filename" followed by a closing right angle bracket (>)
3. enter the text that will serve as the hypertext link in the current document
4. 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.

Relative Pathnames Versus Absolute Pathnames

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:

1. it's easier to move a group of documents to another location (because the relative path names will still be valid)
2. it's more efficient connecting to the server
3. 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.

URLs

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 guide in your document, enter:

 
<A 
HREF="http://osu.orst.edu/admin/web-edit/html/">
Web Editors' Companion Guide</A> 

This entry makes the text Web Editors' Companion Guide a hyperlink to this document.

For more information on URLs, refer to:

• A Beginner's Guide to URLs

Links to Specific Sections

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 Companion Guide 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.

Mailto

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:web@mail.orst.edu">
 Webworks Staff Support</a> 

to create a mail window that is already configured to open a mail window for the Webworks Staff Support alias. (You, of course, will enter another mail address!)

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 s tretching/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="/example.gif" alt = "This is an example picture" ALIGN=TOP>< /samp>). 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="/example.gif" alt="This is an example picture" 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 = "/example.gif"> 
</p> 

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:

• ColorPro Web server
  
• WebColor - downloadable software site
  

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="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.

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