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ASP ADVANCED
One explanation for the
explosive growth of the Internet is the openness of its standards.
HTML was designed to be platform - and browser-neutral.
In theory, a Web page should appear the same way regardless of the
browser and the computer being used.
In reality, however, this was never quite true.
From the beginning, Netscape introduced proprietary HTML tags.
For example, Netscape Navigator version 1.0 could interpret an HTML
tag for blinking text. To
this day, the majority of non-Netscape browsers can't interpret this tag.
Frames are another example of a Netscape extension to HTML. As competition between
Netscape and Microsoft has heated up, the situation has only worsened.
Microsoft has introduced its fair share of proprietary tags.
For example, the <BGSOUND> tag, which plays background
sounds, and the <MARQUEE> tag, which displays a scrolling marquee,
can be interpreted only by the Microsoft browser. HTML is being fragmented
steadily into multiple standards. With
each new version of the Microsoft and Netscape browsers, the gap between
what might be called "Netscape HTML" and "Microsoft
HTML" widens. This
creates a serious problem for the Web page designer. On the one hand, there's
pressure on the Web page designer to include the newest HTML tags.
Never underestimate the "coolness factor." If you want
users to return to your Web site, you need to push the limits of HTML. On the other hand, there's
pressure on the Web page designer to make Web pages universally
accessible. No one
appreciates a Web page that they can't see.
As soon as you use proprietary HTML tags, you risk losing whole
populations of potential users. Caught between these
contradictory pressures, what should a good Web page designer do?
Microsoft's answer is to use the Browser Capabilities component, as
described in the following section. Using the Browser
Capabilities Component You can use the Browser Capabilities component to display different Web pages, depending on the capabilities of a browser. For example, some browsers support frames; others don't. Using the Browser Capabilities component, you can detect whether a browser supports frames, and display a framed version of a page only when appropriate. By default, the Browser
Capabilities component can detect the following features of a Web browser:
To use the Browser
Capabilities component, you need to create an instance of it.
Next, you can simply append the feature you want
to detect to the name of the component instance.
Here's an example that displays a number of browser features: <HTML> Your browser has the
following properties: This script detects and displays a number of browser features. For different browsers, different results will be displayed. How the Browser
Capabilities Component Really Works It's important to understand how the Browser Capabilities component really works so you can understand some of its serious limitations. The component detects the features of a browser by using an HTML request header and a special text file that contains browser information. Whenever a browser makes a
request, it includes a USER -AGENT header in the request.
This header contains information about the type of browser being
used and its version number. You
can retrieve this header directly by using the ServerVariables collection
of the Request object, like this: <%=Request.ServerVariables("HTTP_USER_AGENT")%> When used with Netscape
Navigator 3.0, for instance, the value of the USER -AGENT header would be
as follows: Mozilla/3.0 (WinNT; I) It's important to
understand that this is the only information passed between the browser
and the server. The Browser
Capabilities component depends on the information in this header.
The component doesn't detect any of the features of a browser
directly. The Browser Capabilities
component retrieves the value of the USER -AGENT header and attempts to
match it with a browser definition in a special file named browscap.ini.
The browscap.ini file is located on the server.
When you installed Active Server Pages, this file was automatically
installed as well. The browscap.ini file is
nothing more than a normal text file.
It contains a list of browsers and their features.
For example, this is the browser definition in the browscap.ini
file for Netscape [Netscape 2.0] The Browser Capabilities
component uses this definition when it reports the features of Netscape
Navigator 2.0. You can modify this text file directly.
For example, contrary to reality, you could specify that Netscape
Navigator 2.0 can use the <BGSOUND> tag by changing backgroundsounds
from FALSE to TRUE. There are a number of
browser features that the Browser Capabilities component should detect but
doesn't. For example, it
would be extremely useful if you could use the component to detect whether
a certain browser can use the Secure Sockets Layer or cascading style
sheets. Because this
information isn't included in the browscap.ini file, however, you can't
use the Browser Capabilities component to detect these features.
However, you can add this information to the browscap.ini file
yourself. For example, you
can add the following two lines to the definition entry for Netscape
Navigator 2.0: SSL=TRUE Once these two lines are
added, the Browser Component will report these features for Netscape <%=MyBrow.SSL %> You might notice that
many of the browser definitions in the browscap.ini file look like this: [Mozilla/2.0 (Win95; U)] When a browser definition
has a parent parameter, the definition will inherit all the features of
its parent. The preceding
definition inherits all the features of the Netscape Navigator 2.0
browser. For instance, even
though the definition doesn't specify whether the Windows 95 version of
Netscape can use frames, the Browser Capabilities component will report
that this version of Netscape Navigator can use frames because its parent
can. With the parent parameter,
the same information doesn't have to be entered over and over.
You can make one parent definition, and create a number of smaller
child definitions that contain more specific information.
Any browser feature specified in the child definition that
conflicts with the parent definition will take precedence. The Browser
Capabilities component is only as accurate as the browscap.ini file.
If someone is using a browser or a version of a browser that isn't
included in the browscap.ini file, the Browser Capabilities
component won't be able to report its features accurately. When the
Browser Capabilities component doesn't recognize a browser, it reports the
features specified for the default browser.
This is an example of the default browser definition: [Default Browser Capability
Settings] Again, if you don't
like the default properties specified in the browscap.ini file, you can
modify them directly. For
example, you might not want to assume that all browsers can use tables.
To change this assumption, simply change the value of the tables
property in the definition for the default browser.
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