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2.5 Browser compatibility

Publishing date: May 2002

Web browsers are computer programs that use the Hypertext Transfer Protocol (HTTP) to access data across the World Wide Web.

They interpret the HTML and CSS formatting instructions contained in documents, and display the information with the appropriate structure (headings, tables, images etc) and presentation (font sizes, styles, colour etc).

Although Microsoft Internet Explorer and Netscape Navigator are the most popular web browsers, there are many more available that have been developed often with particular operating systems or disabilities in mind.

Use each checklist to ensure that your web pages comply with these guidelines

2.5.1 Checklist and summary: Core guidance


Any web page must be able to print out legibly on a standard office printer


Web content is constructed with any number of authoring tools and viewed on a wide variety of computers running various different browsers on all manner of operating systems.

Webmasters must ensure that the whole of their web presence is generally available to the widest possible audience. No part of a website should be designed specifically for a particular browser. This will simply disenfranchise users who happen to use other applications software.

2.5.2 What is a web browser?

A web browser is a computer program that allows users to access information across the World Wide Web. The capabilities of different browsers vary and different browsers will typically render a Web page differently according to their capabilities. Many browsers only partially support the latest revisions of recommended mark-up standards including HTML and CSS. Generally, details of what standards and versions of them are supported by which browsers are not easy to find. Annex A Web browser applications has more information on this.

Every information resource accessible on the Web has a unique address or URL. The user typically enters the URL of a ‘web page’ into the browser and the browser locates and fetches the resource. Any other files that are dependent on this file including external script and CSS files, images and other embedded content are also fetched. Sections 2.1.2 and have more details of how this works.

When the browser receives the file, it interprets the HTML and CSS mark-up within it and uses these to format the file for display and renders it as a ‘page’ on the screen or other output device.

Every browser has a number of common features that facilitate the retrieval of information from the web by the user:

Go to Annex A Web browser applications
Go to section 2.1 Basic website structure

2.5.3 Types and versions of browsers

Most government organisations have Microsoft Internet Explorer or Netscape Navigator installed as their standard office web browser used to access their departmental intranet. Typically, the same browser and version of it is installed on all of the desktop computers within the organisation. Because everybody has the same version of the same browser, all users see an identical view of their Intranet content.

Unfortunately these same conditions do not exist on the Internet where a diverse range of browsers of varying vintages are in use. There are many reasons for this. Although the pace of browser development is fierce and competitive between manufacturers, not all users elect to install every new version that is released. Newer releases of browsers tend to work best on contemporaneous versions of their corresponding operating systems and the underlying computer hardware. This may act as a disincentive or even a barrier to updating browsers. Many organisations have deployed a standard platform ‘package’ of hardware, operating system and applications software suite that have all been tested and verified to work together. The complexity and cost of the package integration and testing work is such that it may not be appropriate to install every new browser release that comes along.

Beyond the world of the MS-Windows PC, there are other computers and devices running different operating systems each with browser development strands that are different to the PC MS-Windows ones. Versions of Internet Explorer and Netscape Navigator are available for other desktop PCs such as Apple Macintosh and the UNIX system family. However, there tend to be subtle and not so subtle differences between the way they render HTML and CSS and in their document object models and scripting environments compared with the MS-Windows counterparts.

Devices such as electronic personal organisers and cellular mobile telephones are also increasingly capable of browsing the web. The display screens on these devices are very small compared with those on desktop computers and their browsers use modified versions of the HTML markup language. Consequently it is necessary to build modified versions of ‘standard’ websites to work with these devices.

There are a number of special browsers, add-ons to standard browsers and purpose-built computer hardware devices available to enable users with any of a wide range of disabilities to access the web. See section 2.4 for the implications of the requirement to build in universal accessibility to websites.

Most web users now have access to monitors and display adapters that can display 800 x 600 pixels. Although this can be taken as the default standard it is still important to ensure that your website is legible in the lowest screen size of 480 x 640 pixels.

By the same token, some users still have graphics adapters in their PC that can only display 16 colours. While this should not limit creativity, all pages produced and made public must still be legible and understandable when viewed in this format.

Many PC graphics cards allow users to change their monitor settings down to 16 colours and reduce their viewing capabilities down to 480 x 640 screen size.

Doing this will enable you to see what some web users have to endure when visiting your site.

2.5.4 Example of different browsers

The following screen image captures illustrate the subtle differences in the way a number of browsers display the same web page. The browsers were all running on a Microsoft NT4 operating system with an 800 x 600 pixels screen set in ‘True Color’ mode. It can be seen that the font sizes, the structure of tables and the rendering of bulleted lists have all rendered slightly differently.

2.5.5 Testing with different browsers

The recommendation is that pages on government websites should work with a wide range of browser makes and release versions. HTML version 4 should be used to specify content structure and CSS should be used to specify presentation properties. Client-side scripting (JavaScript) may also be used. However, web sites should degrade gracefully: that is to say they should be usable:

  • with browsers that recognise only prior revisions of the HTML markup language;
  • with browsers that pre-date the introduction of CSS;
  • with all versions of browser scripting environments (Document Object Models);
  • with browsers that do not support client-side scripting at all or that have client-side scripting turned off.

In order to be sure that your web site works properly with a wide range of browsers you should consider testing it (or a representative part of it) with a range of browsers. The following are worth considering for testing with:

Netscape Navigator 1 or NCSA Mosaic 1

These very early graphical browsers cannot render frames, which makes them ideal for testing the noframes elements of websites that use frames. These browsers also do not support HTML tables.

Netscape Navigator 2

The second version of this browser can render a limited number of the frames attributes. Its inability to deal with any JavaScript scripting will raise awareness of any possible problem areas. It does not support CSS.

Netscape Navigator 3

This was the first graphical browser to support the use of frames and some limited JavaScript capability. If a frames-capable site displays well in this browser you can be fairly confident that it will work in most others. It does not support CSS. This browser is available free of charge.

Microsoft Internet Explorer 3

The capabilities of this browser are broadly similar to those of Netscape Navigator version 3 although there are differences in the way specific HTML elements render. Both the MS-Windows version and the Apple Macintosh contain (differently) partial and buggy implementations of CSS. Both versions also have an implementation of JavaScript that was reverse-engineered by Microsoft from the Netscape Navigator 3 version. Microsoft first used the name ‘JScript’ to refer to this implementation of JavaScript. As with the CSS implementation, the MS-Windows and Apple Macintosh JScript implementations are quite different. Microsoft released updates to the MS-Windows JScript implementation for the MS-Windows version during the lifetime of this browser.

Netscape Navigator 4

This is a widely installed version of the Netscape browser. An important point to be aware of with regard to ensuring that website implementations are compatible with Netscape Navigator version 4, is that a there was a large number of minor releases each with different capabilities.

Microsoft Internet Explorer 4

This browser was the first to include a comprehensive document object model (DOM) that gave access to every HTML element (and its attributes) and CSS property in displayed pages via the scripting environment. Microsoft referred to this feature as ‘Dynamic HTML’ (DHTML).

Microsoft Internet Explorer 5

At the time of writing, this is the most commonly used web browser. Any website must render properly on this browser. This browser is available free of charge.


The Mozilla organisation was originally formed by the Netscape Corporation as a way of involving the web software development community in the making a web browser designed for standards compliance, performance and portability. This browser is available free of charge.

Netscape Navigator 6.1 and 6.2

These are the latest versions of the Netscape Navigator browser and is based on the Mozilla organisation’s software.

Microsoft Internet Explorer 6

At the time of writing, this browser had recently been released. It is the first browser to include support for the W3C Platform for Privacy Preferences (P3P) recommendation. The user’s privacy preferences as set in the browser preferences may interact with a website’s published P3P privacy policy in a way that will cause this browser to manipulate cookies served to it. See section 4.7 Cookies.


This is an open-standard web browser designed and distributed by the W3C. It has a number of built-in browsing modes: the same web pages can be displayed in standard graphic mode or in a DOS Lynx emulator mode. Amaya is extremely useful for testing compliance with open standards and accessibility. At the time of writing version 5.2 had been released. This browser and authoring tool is available free of charge.


This browser not only supports the browsing of pages on the World Wide Web but can also render WAP pages in WML. This browser is available for a free 30-day trial period but further use of the browser requires a licence fee to be paid.

Opera Software has recently released their version 5 browser, which although completely free, has a reduced viewing area and displays advertising all the time.


LYNX is a line-mode browser that has been available for many years and is still popular today. It displays pages in a ‘DOS format’: it displays only unstyled text. It remains popular for a number of reasons including because it renders documents very quickly and also interfaces extremely well with Braille displays.

Many of these browsers are also available for multiple operating systems, such as, Microsoft Windows, Apple Macintosh and members of the UNIX operating system family. Web pages will often render differently in the same browser running on a different operating system platform. You should therefore also give consideration to testing on a selection of different operating system platforms. Useful browser resources

Go to Annex A Web browser applications
Old versions of Netscape browser products

2.5.6 Special-purpose browsers

Many different browsers are used for viewing information on the web. The ones mentioned so far in this section are only some of those available.

Some sections of the user community use special-purpose browsers to help overcome a specific disability. It is the webmaster’s responsibility to be aware of these browsers and their specific capabilities and to ensure that the organisation’s website is accessible through them.

The W3C WAI committee has stated:

People with visual impairment or reading difficulties rely on speech output, Braille displays or screen magnification; and in many cases use the keyboard instead of the mouse. People who can't use a keyboard depend on voice recognition for spoken commands, or on switch devices which head, mouth or eye movements can control. People whose eyes are busy with another task may need web access using voice-driven systems.

Go to Annex B Disability-specific web browsers
Go to section 2.4 Building in universal accessibility

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